THE ONE PIECE THAT MOVES AND CAPTURES DIFFERENTLY
For five of the pieces the square moved to and the square captured on are the same square. For the pawn they never coincide anywhere: it goes forward along the file and captures diagonally forward. A pawn standing directly in front of an enemy pawn can therefore neither advance nor take it — a position impossible for any other piece.
Capturing en passant comes from here. The two-square first move entered the rules later than the game itself, and with it came the need to let the opponent take such a pawn as though it had advanced only one square. It can be done immediately and not later: the right disappears the moment any other move is made.
And a third thing peculiar to the pawn: on reaching the far rank it stops being itself. The player must exchange it for a queen, rook, bishop or knight of the same colour, as they choose and regardless of what pieces are already on the board. The rules allow neither leaving the pawn a pawn nor making a king of it.