I like solving puzzles, it's fun and challenging and blah blah blah. Anyway, here are some of the more interesting ones that I've beaten my head against over the past short while. Stars are used to indicate difficulty: 0 (trivial), * (easy), ** (hard), and *** (very hard).

• Three Litres - an easy one to start with (0)
• Dinner at the Smith's - harder, but such a beautiful solution (**)
• Dropping Eggs - I'm not sure why, but mathematicians always seem to have it in for eggs (**)
• A Big Bag of String - this one's great, lots of chances to be off-by-1 or off-by-k (*)
• What Colour is the Bear? - like “Dinner at the Smith's”, but easier (0)
• Sequence Prediction - thanks to Nela Cicmil for this one, you will find it either absurdly easy or extremely hard (*)