Escalator
Inspired by a Twitter problem posed by Bowman Dickson.
How long is the escalator? All you have control over is how fast you are walking, in ft/sec. Is there even enough information to solve the problem?
See the original problem at http://mathhombre.tumblr.com/post/27719574817/escalator
You can set up comparinsons of two different walking speeds, or comparing moving and standing.