It's Obvious...

So two mathematicians meet in the corridor of their building and one asks the other "so what are you working on?" The second mathematician replies "I've been working on this interesting proof, come into the lounge and I'll show you".

The two go into the faculty lounge and the guy starts to work out his new proof. After chugging along for about 15 minutes, he turns to the ~~first~~ second guy and says "so here, you'll see that the answer is therefore obvious". The first guy stares at it for quite while. He makes a few notes on the side of the board, then stares some more. He scribbles more and stares more.

The first guy eventually shrugs and goes home. The first guy keeps working well past midnight. He finally goes home and collapses in bed, but is clearly obsessed with the problem. Over the next few days, he keeps working on it in his every spare moment. He fills the white board in his office, the one he has mounted in the garage at home and he even fills the old blackboard in the abandoned classroom in the old lab. Finally, one day he jumps up, punches his fist in the air and screams "Eureka!" at the top of his lungs. He runs down the corridor, into the classroom where the other mathematician is holding a seminar and yells out:

"You're right, it's obvious!"

Eit: Corrected idiot typo...