Start with giving them [FizzBuzz][1] to solve. That should weed out the worst of them.

Then something a bit harder - for example, how to reverse a string without built in library functions. Ask them to talk while solving in order to see what their thought process is.


 [1]: http://www.codinghorror.com/blog/2007/02/fizzbuzz-the-programmers-stairway-to-heaven.html