iO9, via Nature, has a great article about the physics of shampoo. - why do little drops of shampoo sometimes jump out while you're pouring it onto an angled surface? Sometimes it looks like grease popping off of a hot pan!
When you pour a liquid such as shampoo onto an angle surface, at first the liquid piles up a bit at the bottom of the surface, effectively lubricating that surface.. When new liquid is applied on this lubricated surface, it goes very, very fast and will bounce upwards if it hits a dent or smashes into the end of the surface. The momentum that the liquid builds up is enough to escape the surface!