What is "Bughouse"?

Bughouse is a chess variant played on two chessboards by four players in teams of two. Each team member faces one opponent of the other team.
Partners sit next to each other and one player per team has black pieces, while the other has white pieces.
Each player plays the opponent as in a standard chess game, with the exception of the rules specified below.

Captured pieces

A player capturing a piece immediately passes that piece to their partner. The partner keeps these pieces in their reserve and may, on their turn, instead of playing a regular move, place one of these
pieces on the board, called dropping the piece. Pieces in reserve may be dropped on any vacant square, including squares where the piece delivers check or checkmate; however, pawns may not be
dropped on the first or last rank. Dropped pawns may promote, but all promoted pawns convert back to pawns when captured. In play over the board, a promoted pawn can be put on its side to
indicate promotion. A pawn placed on the second rank (its typical starting square) may move two squares on its first move, and, if it lands directly next to an enemy pawn, be captured en passant. A rook
placed on its typical starting square (a1, h1, a8, h8) may take part in castling. Each player must keep the reserve or stock pieces on the table in front of the board, always visible to all players of the game.