Why do we yawn?

You ask: Why we yawn?

Big open mouth, deep pull of air, jaw stretch wide — that yawn. Old tribe say yawn grab more air when blood run low on it, but science tribe test this and it not hold up. Leading idea now: yawn cool a hot brain. Wide jaw-stretch push more blood up top, cool air rush in, and brain temperature drop a little — sharp brain like cool brain. That why Caveman yawn when tired or just waking, when brain switch gear. And when one tribe-mate yawn, Caveman yawn too — about half the tribe catch it — this catching yawn tie to feeling-with-others, a mirror in brain. Science still argue the whole story. Cave wisdom: yawn most likely brain splashing cold river on itself to stay sharp — and catching yawn show Caveman feel with tribe.