Try to Avoid Moving the Same Piece Twice

Chess Question:
Why can’t the queen take on d5?

Solution:
If the queen takes, black will recapture with
its queen. White cannot take exd5 because
the pawn is pinned.

Chess Question:
What happens if white captures the pawn on d4?

Chess Question: Why does Bxe3 lose?

Why should we avoid moving the same piece twice in the opening?:
The sin of moving the same piece twice in the opening with Ng5 lets your opponent get more of his pieces out. Notice that Morphy was black but because of the disregard of this principle black is able to get more pieces out faster.