chessGMA fork is one move that attacks two or more pieces at once, so the opponent can save only one. Knights are the classic forking piece, but queens, pawns and even kings can fork too. There are 1,539 puzzles with this theme on chessGM. Every puzzle is rated, so the ones you get match your puzzle rating, and after solving one you can play through the whole solution on the board. It is free and needs no sign-up.