http://www.todaysbigthing.com/2010/02/02
I attempted this the other day. My girlfriend took a lot of convincing (she can be pretty skeptical of things like this) but after a little convincing she bit, hook, line and sinker. The trick is get her to move her hands reflexively, because she'll forget all about the water over her head. Go in for a tit slap, or tickle a little. Just make sure you get out of the way or you'll get soaked too.
Also the stronger your girlfriend is the more water you can put in the container, lol. Just be sure to do it at her place...not that it matters (if you're near her it should be in the kitchen, and the mop should be close by for her)