I'm not sure how much you know already, so I'm just going to go through this step by step and hope that I cover everything.
First of all, wood comes from trees. Trees are pretty much just really big plants.
So, let's talk about plants. As you may or may not know, all living
things are made up of cells. There are two types of cells. "Animal
cells" are, not too surprisingly, the type of cells in animals. "Plant
cells" are the type of cells in plants. They are made up a little
differently, so I'm going to talk about plant cells.
The inside isn't too important to this question, so I'll focus on
the outside. Each plant cell has what is called a "cell wall" - this is
pretty much just what it sounds like, a firm 'wall' around the cell.
This wall, however, isn't completely solid - certain things, like
water, can get through it.
For a picture of a plant cell, look here
The cell wall is in blue around the outside.
Plant cells are pretty much designed to absorb water. If you want
to do an experiment with this, take a limp piece of celery and put it
in some cold water. You will notice that after a little while, the
celery firms back up. This is because the celery cells absorb the water
(they get a little bigger, which is what makes the celery firm up). The
water is absorbed through a process called "osmosis" - this is just a
fancy science term for water moving through a membrane, like the cell
So, wood cells absorb the water into themselves, which is why wood can soak up water.
Hope this helps!
(republished on 07/18/06)