What's the minimum number of squares you have to remove from a chessboard, so that if you put a knight down on any remaining square, he won't have any valid moves?

I am always looking for new puzzles! I've done lots and lots of them (and failed at just as many), and I'm ever on the hunt for new challenges.

By logic puzzles I don't mean the ones you can solve with a grid, the "Adam lives on 1st St. in a house that's either green or blue, Betty lives in a red house on 2nd street, the person who lives in the yellow house has a dog, etc." I'm looking for more freeform logic puzzles, mystery puzzles, math puzzles, etc. Examples of classics would be the "Wolf, Goat, and Cabbage" puzzle and the "Measure four gallons with a five gallon and a three gallon container".

I'd love to respond in kind with puzzles that I love, or I can offer simbi for puzzles I haven't seen before. Mega bonus points if you like inventing puzzles as much as you like solving them!