When I interview programmers I start off with this question:

Write a function in C (on the whiteboard) that takes an array of integers and returns its sum.

I explicitly explain to candidates that this is a warm-up question to start the interview, and that I'm looking for the textbook solution, no tricks.

The advantages are: 1. It takes a reasonable candidate 3 minutes to write down the answer. 2. Answering a question correctly gives them confidence, always important in an interview. 3. It provides a good basis for a little advanced discussion (overflow, speed, error handling, supporting infinitely large numbers, etc).

Amazingly enough, some candidates have actually failed this...