Sweatdrop & others
For the most part, I'd say it's a combination of "who knows" and "tradition". Basically, though, once an image gets used enough, it becomes accepted as the way to depict that, just like with eyes bugging out or the pounding heart image that's used in American cartoons to show men being excited by a pretty girl. In a lot of cases, even the people drawing it probably don't know why.
The rivers of tears, for example, are just a way to show somebody crying without going into a lot of detail. I can't say for sure, but they probably evloved slowly from people exaggerating the way tears are drawn, eventually reaching where they are now--a simplified, "purified" form that everybody recognizes as crying uncontrollably.
The mushroom-shaped white puff almost certainly developed from manga artists drawing a billowy little cloud to illustrate somebody sighing (since exhaling doesn't exactly make a sound). That probably got simplified and turned into a little mushroom shaped icon over the years, which then translated into animation.
Other cases come from Japanese tradition. It's a folk tale that men's noses bleed when they get aroused, because of increased blood pressure (heart pumping, whatever). Hence the nosebleed.
I don't know about the nose bubble, but it's probably a visual representation of drooling to show somebody is sleeping soundly (why do sleeping people have a string of "Z"s coming out of their head in American cartoons, after all?). The tiny pupils are just designed to show shock--basically, since anime characters eyes are already huge, the best way to further exaggerate them "bugging out" is to make the pupils smaller.
Chibis/SD is a logical progression to the cutest possible form. It's scientific fact that humans find things with big heads (relative to their bodies), and large eyes cute (that's what babies look like), so if you extend that in the extreme (sice anime chacters already have unrealistic proportions), you get SD. It has come to be used by convention, of course, to illustrate that whatever is going on should'nt be taken seriously--they're just silly "cutified" versions doing it.
As Tenseiga said, the giant "Say what?" sweatdrop is a little different from just sweat illustrating somebody is nervous.
Frequently, people are shown sweating due to being nervous--that makes perfect sense, even though the drops of sweat are usually drawn large so they're visible in a simple frame.
That probably developed, as Guardian suggested, into sometimes drawing one larger drop to save time (though a small drop beside the eye is also used frequently). Then, by extension, also explains why sweat is sometimes drawn on the back of someone's head--the artist wants to show they're shocked/nervous, but they're facing the other direction, so they just draw it anyway.
But that doesn't exactly explain the giant floating sweatdrop, or why people "sweat" when someone else says something shockingly stupid.
I talked it over with Akemi, and she thinks the former (big, cartoony drop) was developed by some particular artist relatively recently (last 20 years or so)--she doesn't remember ever seeing that when she was a kid, and thinks it popped up abruptly at some point.
In the case of the shock-sweat, she thinks that by Japanese standards the reaction sort of makes sense (sweating, that is). If you want to read too far into it, you could probably chalk it up to the fact that Japanese culture promotes empathy and group harmony, so someone else saying/doing something terminally stupid and embarassing carries over a bit to the person listening, causing a bit of empathic embarassment/nervous shock (though I sort of understand the feeling, so it's not entirely Japanese), and thereby causing sweat. Add that to the giant cartoon sweatdrop developed in manga, and you've got your sweatdrop.
Mostly speculation, but there's a theory.
First, one has to remember that all those visual expressions come from manga. When you have to express an emotion in one frame, it's not difficult to understand that sometime the mangaka would resort to "tricks" or exaggeration.
I think the origin of the sweatdrop has pretty much been pined down my Makosuke. In the Japanese culture, the embarassment of others extends to yourself, causing nervousness and therefore sweat. Originally it was probably just a little drop on the side of the head, but with the exaggeration common in manga, it evolved (or suddenly mutated) into the huge form that we know today.
I'm surprised nobody got the nose bubble thing. This is actually something that happens in the real world. Mucus will accumulate in the nose, block the entrance of a nostril and, with the person's breathing, create a bubble... it's just like bubblegum! It's pretty rare, but I think it's more common in babies. It's associated with very deep sleep.
The rivers of tears, for example, are just a way to show somebody crying without going into a lot of detail. I can't say for sure, but they probably evloved slowly from people exaggerating the way tears are drawn, eventually reaching where they are now--a simplified, "purified" form that everybody recognizes as crying uncontrollably.
The mushroom-shaped white puff almost certainly developed from manga artists drawing a billowy little cloud to illustrate somebody sighing (since exhaling doesn't exactly make a sound). That probably got simplified and turned into a little mushroom shaped icon over the years, which then translated into animation.
Other cases come from Japanese tradition. It's a folk tale that men's noses bleed when they get aroused, because of increased blood pressure (heart pumping, whatever). Hence the nosebleed.
I don't know about the nose bubble, but it's probably a visual representation of drooling to show somebody is sleeping soundly (why do sleeping people have a string of "Z"s coming out of their head in American cartoons, after all?). The tiny pupils are just designed to show shock--basically, since anime characters eyes are already huge, the best way to further exaggerate them "bugging out" is to make the pupils smaller.
Chibis/SD is a logical progression to the cutest possible form. It's scientific fact that humans find things with big heads (relative to their bodies), and large eyes cute (that's what babies look like), so if you extend that in the extreme (sice anime chacters already have unrealistic proportions), you get SD. It has come to be used by convention, of course, to illustrate that whatever is going on should'nt be taken seriously--they're just silly "cutified" versions doing it.
As Tenseiga said, the giant "Say what?" sweatdrop is a little different from just sweat illustrating somebody is nervous.
Frequently, people are shown sweating due to being nervous--that makes perfect sense, even though the drops of sweat are usually drawn large so they're visible in a simple frame.
That probably developed, as Guardian suggested, into sometimes drawing one larger drop to save time (though a small drop beside the eye is also used frequently). Then, by extension, also explains why sweat is sometimes drawn on the back of someone's head--the artist wants to show they're shocked/nervous, but they're facing the other direction, so they just draw it anyway.
But that doesn't exactly explain the giant floating sweatdrop, or why people "sweat" when someone else says something shockingly stupid.
I talked it over with Akemi, and she thinks the former (big, cartoony drop) was developed by some particular artist relatively recently (last 20 years or so)--she doesn't remember ever seeing that when she was a kid, and thinks it popped up abruptly at some point.
In the case of the shock-sweat, she thinks that by Japanese standards the reaction sort of makes sense (sweating, that is). If you want to read too far into it, you could probably chalk it up to the fact that Japanese culture promotes empathy and group harmony, so someone else saying/doing something terminally stupid and embarassing carries over a bit to the person listening, causing a bit of empathic embarassment/nervous shock (though I sort of understand the feeling, so it's not entirely Japanese), and thereby causing sweat. Add that to the giant cartoon sweatdrop developed in manga, and you've got your sweatdrop.
Mostly speculation, but there's a theory.
First, one has to remember that all those visual expressions come from manga. When you have to express an emotion in one frame, it's not difficult to understand that sometime the mangaka would resort to "tricks" or exaggeration.
I think the origin of the sweatdrop has pretty much been pined down my Makosuke. In the Japanese culture, the embarassment of others extends to yourself, causing nervousness and therefore sweat. Originally it was probably just a little drop on the side of the head, but with the exaggeration common in manga, it evolved (or suddenly mutated) into the huge form that we know today.
I'm surprised nobody got the nose bubble thing. This is actually something that happens in the real world. Mucus will accumulate in the nose, block the entrance of a nostril and, with the person's breathing, create a bubble... it's just like bubblegum! It's pretty rare, but I think it's more common in babies. It's associated with very deep sleep.