Basically, it suggests that if you drop enough needles (or matchsticks) of length l on a wooden floor with planks of width 2l, you can take the ratio of total number of matchsticks dropped to the number of matchsticks that cross the crack. This ratio should eventually equal pi (3.14159265).
I thought I'd write a C2 program to simulate the proposition, and it worked pretty well.
EDIT: dropbox hyperlinks aren't working for some reason, so try copying and pasting these links.
Here's a demo.
Here's the capx.
If you let the calculation run for a little while, you should see the ratio value approach pi. For a variety of reasons it won't be perfect but it gets pretty darn close.
It's a very interesting proposition, and its solution goes into the depths of probability calculus.
So, there's your uncomfortable math truth for the day!