What's the best science-related quote?

I enjoy wheeling out the odd science-related quote, and would be interested to collect more. To be really great, I think a quote like this needs to be pithy, funny... and make you think. Do you have a favourite? (Please ensure they are from a reliable source.)

It's hard to go wrong with Rutherford's famous:
All science is either physics or stamp collecting.
Which is particularly useful to wind up biologists. I am also rather fond of Konrad Lorenz's advice, which rather a lot of scientists could do to consider:
It is a good morning exercise for a research scientist to discard a pet hypothesis every day before breakfast.
But the one I'll leave you with this morning, which I shall dedicate to cosmologists and string theorists, is:
There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.