We know the mouth is a useful orifice for venting our feelings: if we're "hot," speaking our anger can help us "cool off". And so the bigger the mouth, the better the cooling, right? Actually, yes.
When our brain gets hot, we cool it through the mouth, and the best way to cool through the mouth is by yawning.
Researchers showed this by cooking parakeets.
Okay, they stopped short of actually cooking them, but they found that when temperature increased, the parakeet yawn rate doubled (there was no description of researchers' yawn rates).