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I have downloaded a description for a random (relatively simple) algorithm off Wikipedia before, put it as the comment in a text file, and then asked people to implement it.

Do it yourself first, so that you have interesting questions to ask; and you can keep iterating on it too. Get them to write tests for it, documentation, what are the error conditions, how do they want to do error processing, and so on.

I used to ask them to find average of array of ints. For bonus points to make code bullet-proof (what if someone passes null array, what if integer overflow happens etc). Surprisingly how many candidates failed because they used integer division (in Java), and how many didn't even think that something can be null...

A computer science or maths degree used to do the trick, but nowadays many such graduates just know how to create a java dialog box in Eclipse.