Jim saw this simple experiment the other day when he was watching Beakman’s World (a kid’s science TV show) with the boys. It is quite counter-intuitive and really simple to do. All you need are 2 drinking straws and a glass of water (we used juice).

Begin by putting one drinking straw into the beverage and having the child take a sip. Nothing unusual here.

Now tell the child you are going to give them a second straw, but instead of putting it in the beverage, you want them to put it outside the glass. Ask them what they think will happen when they sip from both straws—one sucking liquid, the other air.

If all has gone right (the child is not blocking the second straw with the tongue or resting it on the table), there is no liquid in the child’s mouth. You can play this up a bit, telling them to suck harder, switch straws, etc. Make it fun. :-)

Now switch kids and let the older have a shot at it and see if he can get any liquid to come up with the second straw sucking air.

Lastly, have the children place both straws in the beverage and sip, so they know you weren’t using any kind of trick straw. This is science, not magic or a practical joke.

So what’s the explanation here? Didn’t you think that when sucking both liquid and air the child would get a mix of each? (I did!) But no. Sucking on a straw does not work the way many of us think. We aren’t so much sucking up liquid as creating a vacuum, which then the liquid rushes into as the air pressure on the rest of the liquid in the glass forces it along. By sucking air, you are not creating a vacuum, hence the straw will not work.

This also explains why if you’ve ever tried to drink from a straw with a small hole in it, it works until the hole gets above the level of the liquid. Once the hole is in the air, there is no longer a vacuum, and the straw seems to stop working.