Twenty-one simple but mystifying tricks--with such titles as ""The Mystic Year,"" ""Tricky Triangle,"" and ""Crystal Calculator""-involving addition, multiplication, drawing or crossing out lines, odds and evens, number squares, etc. The better ones are based on mathematical peculiarities, a few on tricky peeking or preliminary memorization; in any case the trickster will amaze his friends by guessing the complicated answer or performing the impossible feat. For kids still without pocket calculators to play with, it's a reminder that using your own head can be even more fun.