Motion sensing with a single IR sensor?

[Half Shell]

I've been seeing a couple of projects using motion sensing but they're only using a single sharp ir sensor. I've been playing with mine but can't figure out how to do motion sensing with a single IR sensor. Is there a trick I am missing?

[cooldog]

http://www.societyofrobots.com/step_by_step_robot_step4.shtml

this uses motion to follow a object

[Admin]

If I were to do it, I'd have it scan and save the distances in an array.

Then I'd have it scan again.

Do a comparison between those two scans, and if it finds a big difference in values, you know something moved and the angle wrt the robot of that movement.