Question about a changing oil level

[Goose]

This is a weird one, and I though somebody might have some idea of what might be going with this situation...

I recently checked my oil level one morning (cold engine) and saw that the oil level appeared to have dropped - not much, maybe 1/4 of the distance between the upper and lower marks on the 'stick. My first reaction was, "oh crap, my engine's burning oil". The car was on a level surface and had been idle for hours. To be double-sure, I even unscrewed the oil filler cap and waited a while - just in case there was some kind of "vacuum" that might prevent all the oil from draining down to the bottom of the engine.

I mentioned this to a friend and they suggested that I check the oil when the engine is warm. When checking the warm engine (it sat for about ten minutes), the oil level was perfect.

The car was in the same spot both times, so no uphill/downhill change.

I can't believe that motor oil expands that much when warm, but it's the only explanation I can think of for the change. If anything, I would think the level would drop on a warm engine due to oil still being up in the head/etc.

Can anyone explain this?