His career has shed light on the secrets of the universe, from the nature of space-time to the workings of black holes, but there is one conundrum that still baffles the world’s most famous scientist.

In an interview to mark his 70th birthday this weekend, Stephen Hawking, the former Lucasian professor of mathematics at Cambridge University, admitted he spent most of the day thinking about women. “They are,” he said “a complete mystery.”