This experiment uses perspex tubes which are clamped to a board, and can be filled with water. When the bung at the bottom of a tube is pulled out, it empties. If there is a bung in the top, it empties much more slowly, as the air has to get in the bottom (a 'slug bubble').

What to do.

Put a bung in the bottom of a tube.
Fill it up with water using funnel and jug. Kids really want to help fill them up, and this is a good way of getting them involved.
Ask what they think will happen when the bung is removed: does the water rush out, drip out slowly, stay in etc.?
Ask if they think it'll come out fast enough to get their shoes wet.
Then ask if they're willing to test their hypothesis in reality, or take a few steps back and remember where their shoes were at the time (I normally look at the parents for confirmation of feet wetting next!)
Count down and remove the plug. SPLASH :-)
What happened?
Water rushed out etc...

Why?
Gravity pulled the water towards the ground and when the water came out of the tube, and air replaced it from the top. (Notice the sublte hint for the next stage.)
Get it filled it up again, but this time put a bung in the top as well.
WARNING: Make sure you hold the bottom bung on when you out the top bung in as water is not particularly compressable, and you can easily force the bottom bung out by pushing the top one in.
Now ask them what'll happen.
Then demonstrate - you can ask one of them to remove it this time :-)
Did they see the bubble go up the tube? Get them to try to explain what was going on. My explanation is just that for water to come out of the tube, air has to get in, as there can't be nothing in the tube (vacuum) as the high air pressure around will not allow it. However, it did not all stay in there because the hole at the bottom was nice and large allowing air and water to swap. You can see the air going in - it goes up the middle in the form of a big bubble, called a 'slug bubble'. The water flows out down the sides.

Next we have a race...
Fill up all the tubes, put bungs in both ends, and recruit some help to get it started. (You will probably have to loosen off the bungs in the bigger ones, as they are quite difficult to remove.
Which one is going to empty fastest? Why? See what each spectator thinks - often people have different views. They can be encouraged to argue it both ways: perhaps the big tube will empty faster, as there is more space at the bottom, perhaps it will empty more slowly, as there is mor water to come out, or perhaps these effects will cancel out and they will empty at the same speed.

Ready, steady, Go!

The biggest one empties fastest. Why did it do that?
A smaller hole at the end means that there is even less space for the water to swap over, so it comes out much slower. The more space there is for the air and water to squeeze past one another, the faster it can come out.

If you like, you can then move onto some straws, saying that straws are exactly the same, but they have even smaller holes at the bottom.
Straw under water, thumb on the top. Remove from water, remove thumb. A straw has such a small hole that the surface tension of the water cannot be broken.

An interesting addition
[Stolen from the Independent (2/8/06)]
Poke a hole in the side of a cup, cover the hole, fill with water, uncover the hole: water comes out Repeat, only this time drop the cup at the same time as uncovering the hole: water stays in, so you can talk about gravity acting on water and the cup, and things falling at the same speed etc. Probably not a tremendously exciting addition, but maybe something to talk about with very interested people, or something... [Plastic cups tend to shatter when dropped on concrete, and are a pain to make holes in, so maybe paper would be better.]