I always loved this physics problem. I first saw it in the University of Chicago's book of problems for the graduate-school comprehensive "qualifying" exam. The reason I like it is that it can be solved with freshman physics.
Here we go. We have a yoyo resting on a horizontal surface. Assume the picture is accurate. Someone pulls gently on the string, as indicated by the arrow. Will the yoyo:
a) Roll right, in the direction of the pull (wind itself up)
b) Roll left (unwind further)
c) Cannot say without the ratio of the inner and outer radii
d) Cannot say without the value for the mass and the coefficient of friction
No credit without an explanation.