Describe a puzzle, quiz or brain game that you enjoy

A brain game I enjoy is Sudoku. If you haven't tried it, it's a number puzzle played on a grid of nine by nine squares, and the goal is to fill every row, column and block with the digits one to nine without repeating any.

I first discovered it about five years ago, when I picked up a free newspaper on the metro and noticed a puzzle on the back page. I didn't understand the rules at first, so I looked them up on my phone and spent about an hour on the easiest one. I got stuck, of course, and had to rack my brains to find the logic, but when the last number fell into place I felt ridiculously proud. Since then, I've become a bit addicted. Now I do one or two puzzles each day on my phone app, usually on the commute or just before bed. I've worked my way up from the easy level to what the app calls 'expert'.

The reason it appeals to me is that it's very pure. There's no luck involved at all, just pure logic, and each puzzle has exactly one solution. Sometimes I stare at the grid for ages, and then the penny drops and I see a pattern I'd missed, which is very satisfying. It's also a good way to keep my mind sharp, and I find it calming because it blocks out other worries for a while. I like the idea that it's a mental exercise, a bit like going to the gym for the brain. My father has started doing them too, and we compare our times.