Gliding Cup..

I was just sitting here listening to the radio and started screwing around with a cup that had melted ice in it. A small puddle of water was on the desk from the melting ice condensation and the cup was in the middle of the puddle... I tapped the cup and it softly glides across the water puddle..

I think.. How neat.. and tap it again, and the next thing I know its 5 mins later and I'm still tapping the cup and its still softly gliding from one side of the puddle to the other..

Then I see a pattern.. Everytime I tap the cup, it moves away from me then stops and comes back to me.

Can anyone explain this? I'm not good with physics and all, but how is it possible that something thats gliding with very low friction make a full forward progressive stop, then reverse the direction with no added force?

At first I thought it was just my computer desk leaning slightly forward.. so I tried it on several other table tops and everytime it worked..

Maybe my entire house has a slight lean or something Someone else pour a small puddle on their table and push an empty cup (something light, like a empty Mcdonalds super-sized Coke cup) and see if the experiment works for you.. I'm actually really intrigued by this..

If someone has an explanation that requires a less.. messy.. conclusion, just lemme know

I would guess that the water had strong enough "skin" (From something it picked up from the table, be it dirt or cleaning solvent) that when you pushed the cup, the force hit the edge of the water on the opposite side, and bounced back.

And since it takes very little water to float something (only takes like 1/3 an inch for a car to hydroplane, and it weighs a ton) the "current" moved the cup back with it.

It could also be that the table has a slight slant, but wouldnt the water run off then?

Oh well.

I used to do a similar thing with a Soft Drink Can partly full, on a small bit of water on a linoleum table top.

I saw it on Mr. Wizard a science show on in the eraly to mid 80's. If you shake the can enough in a circular motion then let it go while the contents are spinning, it moves on the slippery surface in a funny pattern.

-ADHDsux4me