I had the privilege of taking one of Strang's classes a few years back. He is still incredibly sharp, and has a great sense of wit.

During one lecture, he was half way through solving a problem on the board when he jumped straight to the final answer. He then turned to us, and with a deadpan delivery said:

"We haven’t proved it, but that’s okay, we only live so long."

And then he moved on to the next problem.

I've quoted that line a lot since then.