Guess what's the only thing that has stumped Stephen Hawking throughout his 70 years?
A great mystery
The cosmos explained? Child's play. Black holes and the space-time continuum? Been there, done that. The answers to life, the universe and nearly everything? Now you're talking.
There is very little in this world and beyond that would leave Stephen Hawking struggling for an explanation. Except it turns out, for one thing. And it has puzzled men since the dawn of time: women. "They are," he told New Scientist magazine at the weekend, "a complete mystery."
It turns out that the former Lucasian professor of mathematics at Cambridge University, like most men, spends his days thinking about the opposite sex. Even at 70, which he turns on Sunday.
Mr Hawking is in good company. Philosophers, artists and musicians - they've all tried to unlock this mystery. Many, of course, have failed.
Perhaps it is because men, like Mr Hawking himself, can easily be defined by what they do, whereas women defy categorisation. Maybe it's because while he, and countless other men, strain their minds to explain the mysteries of life, women actually get it.
Or maybe, simply, most men have their priorities wrong.
"Put your hand on a hot stove for a minute, and it seems like an hour," Albert Einstein said. "Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity." It's also a pretty lame pick up line.