Why Qxd5 instead of Rxd5? This puzzle on chesstempo caught my eye. We play as Black:
[FEN &quot;1r1r2k/p4pp/2q/2p1p2P/P1P1Pn/B3NP/4Q2P/R6K w - - 0 1&quot;]

1. Nd5

The White queen is under attack, yet one shall not rush and capture it. The White may fork the Black king and queen and then fork two rooks:
[FEN &quot;1r1r2k/p4pp/2q/2p1p2P/P1P1Pn/B3NP/4Q2P/R6K w - - 0 1&quot;]

1. Nd5 Nxe2 
2. Ne7+ Kf8 
3. Nxc6 Ke8 
4. Nxb8 Rxb8

So, we lose a rook here. Therefore, we need to capture the knight. I took the knight with the rook:
[FEN &quot;1r1r2k/p4pp/2q/2p1p2P/P1P1Pn/B3NP/4Q2P/R6K w - - 0 1&quot;]

1. Nd5 Rxd5

Yet, chesstempo wanted me to capture it with the queen:
[FEN &quot;1r1r2k/p4pp/2q/2p1p2P/P1P1Pn/B3NP/4Q2P/R6K w - - 0 1&quot;]

1. Nd5 Qxd5
2. cxd5 Nxe2

So, why do we play Qd5 instead of Rd5? Besides, does it matter for the White whether to play cxd5 or exd5 on the last board as the second move?
After 1...Rxd5 2.cxd5, both queens are attacked. They either both get captured, or (if black moves his to safety) neither is. So black lost the exchange.
After 1...Qxd5 2.cxd5 Nxe2, white has lost a knight.
Cxd5 seems the better capture to me on general grounds (repairs white structure a bit, makes pawn c5 weaker) but that's far less important than the full rook difference between 1...Rxd5 and 1...Qxd5.