Here's the first surprise: "Debit or credit?" is actually an unfair, misleading question. There is no such thing as a debit card that's used as a credit card. When you hand over a debit card, you are engaging in a debit transaction no matter how you answer. When clerks ask this question, they are really asking you to pick one of two ways they can process your debit – a PIN (personal identification number) based transaction or as a signature-based transaction. One costs the merchant a little more and one takes a little longer to hit your checking account, but fundamentally a debit transaction is a debit transaction.

That's why I use my American Express as much as I can.