I used to do this all the time. I remember it especially associated with riding horses, probably because I was concentrating on not falling off. 
-Lil
A lot of people stick their tongue out when concentrating:
http://www.lovejunk.co.uk/photos/milestone20-12-01/ms3.jpg
http://gelbart.streams.com/gelbart/images/still8.jpg
http://silverdale.cksd.wednet.edu/Staff/Kelso/2004_0314_Breanna_Concentration_Tongue.jpg
http://www.instinctive.co.uk/hcpt/gallery/Just%20For%20Fun/2003/images/cscf0001.jpg
http://www.corless-clement.com/emmapages/emmapictures/199912/tongue.jpg
http://buffononline.com/forum/uploads/post-11-1101224254.jpg
Same here.
Way back when, an uncle and aunt of mine bought a few acres and started faming. Shortly after that, Uncle Leo’s tongue started to hurt. After a long summer of cutting back on cutting back on the jalapenos and whatnot, someone noticed that he would stick out his tongue whenever he plowed. He used a mule, and it was just a thing that he did while concentrating on a tough task.
I do this still in my 60’s, and I can remember being kidded about it as a young lad. But I’ve never starred in a cartoon.
Many people do this. My mother used to say, “you can’t concentrate unless you hold your mouth right.” Kids often suffer injuries from biting the tip of their tongue playing sport.
Uh, on reread, I should have mentioned that Leo’s tongue was getting sunburnt.
Sorry. :smack:
And really hard to draw, too, I bet.
(Although it’s probably even harder to draw a cartoon of someone who is blinking and breathing)
There’s a line heard among people who work with tools about certain delicate and difficult tasks: you can do it if you hold your tongue right.
I believe you see this more in comics than in animated cartoons. There’s a reason for things like this in comics - with a limited space to get an idea across they tend to rely on a visual shorthand. I’ve been making a list of these lately - things like a boy running away from home with his possessions tied in a cloth at the end of a stick, or a pregnant woman knitting booties. The image allows the author to communicate an idea in a single frame without using words.
I don’t think the tongue-sticking-out image indicates determination. Rather, it indicates concentration, especially on a delicate task like threading a needle. It would be appropriate to use it for someone trying to pick a lock (for example), but not for someone trying to lift a heavy object. The stuck-out tongue is usually accompanied by a furrowed brow and beads of sweat flying off the character’s head.