Lesson
White to move. Main line: 1. e4 Rd6 2. exf5
After 1.e4! the pawn attacks the rook on d5 and the knight on f5 at the same time. The rook is worth more, so it must move — and 2.exf5 snaps off the knight, winning a piece for nothing.
The lowly pawn is the most cost-effective forker on the board: whatever it takes, you come out ahead. Keep an eye out for two enemy pieces standing on the two squares a pawn attacks diagonally.