you know you're exhausted when...

... you say 'lets play a few quick rounds of scrabble.' because you normally have a tendency to think yourself funny (this is not due to tiredness), you erupt into a fit of giggles, which turn into some cross breed of giggles, hiccups, and snorts. which cause your airways to close up, leaving you short of breath. during your little outbreak, you discover that your face is wet - due to the condensation streaming out from under your eyes.

that is when you know you are much too tired.