Hi, this is David Sullivan for Chess NYC. I want to talk about legal and illegal moves. Here we have a very basic position, mating position, each side has a king, 3 pawns, and a rook, and it's white's turn to move. White can bring the rook all the way down to the C8 square and say check to black, and this is checkmate. The rook is saying check to the king. The king who goes one square at a time can't move to a place where the rook cannot still get at it. Nobody can block and nobody can capture the checking piece, but often when I'm working with young people they're not totally clear about how pieces move, so when they give an answer like rook C8 I might say, "Well, can't the rook just capture your rook?"
Then I wait to see who is alert and hopefully someone will notice that a rook does not move on the diagonal, so this rook is not allowed to capture that rook. That's breaking the rules. I might say, "Well, okay if my rook can't take my rook can't I just bring my pawn back here like this to block the check?" Then I'm waiting for the child to notice or remember that pawns cannot more backwards so this would be another illegal move. Then last but not least I might say, "If you give me this check maybe I can bring this rook back here and block the check." If the rook comes back and blocks the check you're also breaking another rule. What rule is that? Rooks are not allow to jump. Perhaps I might also have thrown in, "Can't my king take the rook?" That of course breaks another rule, kings are only allowed to move 1 square at a time. Rook to C8 is checkmate. Black cannot make any illegal moves, game over.