The binary tree question has a short definition, requires only knowledge that any candidate will have, and is easy for any logical thinker.
If I am administering a practical test for a junior programmer position, as in, a fresh graduate, the binary tree test is a great one.
I put them in front of an IDE, sit with them, and ask them to write a unbalanced binary tree container with insert, find and delete functions.
Watching a candidate do this tests basic implementation skill, API design, debugging and testing methodology. By the end of it, I have a pretty good idea if they will make a good hire.