"what's the mechanism for absorption and how do you explain it to a pack of 4 year olds?"
Uh.....
I knew what absorption IS, of course -- but how does it work? Internet turned out to be less help than I thought it would be. Dictionaries informed me that absorbency was "the property of being absorbant" (great. A circular definition. Everyone's favorite.) Absorbant was "the ability to absorb" (I was beginning to be suspicious at this point). "Absorb" means to "suck up" or "take up". I wasn't sucking up much knowledge from that.
The other dictionaries and encyclopedias were equally unhelpful. There's a boatload of tests for absorption, but no good explaination. After reading a number of articles, I eventually tracked down the answer -- an answer that's both interesting and surprising:
The surprising answer is that water is sticky.
That's right -- it' s sticky. That answer seemed contrary to me until I explored it more thoroughly.
Stickiness is a physical property of things (ball bearings, for instance, are not sticky but the grease they're covered with is sticky. In the case of water, it's explained by a property called "cohesion" -- the electromagnetic forces that bond molecule to molecule. Because water molecules like to stick to each other (to "hold hands", as someone said) and water tends to "hang out" in globs and drops.
Think about it this way: sticky things stick to other things. If you put a metal spoon into a basket of beads, none of the beads stick to the spoon. Beads aren't sticky. If you put it into a cup of salt, salt won't stick to the metal because salt's not sticky. But if you put it into honey, when you pull it out there's honey on the spoon... honey's sticky. Oil sticks to the spoon.
So does water. We don't think of it as a sticky material because we use it to clean surfaces. But its stickiness... its tendency to "hold hands" with itself and with other substances gives water its most important properties: the ability to "wet" things and the ability to "soak into" things. It "slides into" small spaces and hangs around there (being very friendly), and adds to the weight of the object.
A good article on this is: http://www.exploratorium.edu/ronh/bubbles/sticky_water.html
Scholastic has a very nice little article for parents with some activities that will work for teachers or for curious adults who can't resist the chance to play with some sponges (that would be me!)
http://www.scholastic.com/earlylearner/age4/learning/sponges.htm
