• Cookie sheet
• Pencils
• Tape
• Plastic wrap
• Sink or tub
• Water
• Small scraps of paper or Styrofoam (optional)
  

Fluids, such as air and water, change speed as they flow between and around objects. To see how this happens, build a tiny stream channel. Tape pencils to a cookie sheet so that they make a channel that starts out wide and then narrows. Drape the pencils and cookie sheet with plastic wrap; this creates a waterproof channel. Now barely tilt the cookie sheet against the sink and slowly pour soapy water into the channel.

Does the speed of the water change? How? When?
(Hint: You may want to add small scraps of paper or Styrofoam to the water to help you observe the current's flow.)

What's Going on? The Simple Explanation

Air is pretty pushy stuff. It never pulls or sucks; it pushes. Air is pushing on you right now from every direction. We're so used to air being around us that we often don't notice it. This constant push of air is called air pressure. It allows us to breathe - not a bad thing! Now think about what was happening in the activities you just finished.

Why did the balloons come together when you blew between them? Why did the paper lift up when you blew over it? Air must be pushing these things, but how?

Even before you blew at the balloons, they were surrounded by air pressure. If you tried blowing between them, you disturbed this push in a very special way. How?

Think about this: Either the air between has stopped pushing as hard or the air on the outer sides is pushing harder. Which do you think happened? Which air did you disturb, the air between the balloons or on the outer sides of the balloons? Can you figure out what happened with the paper?

Now you know that the paper was surrounded by air pressure. How did you change the air when you blew over the paper?
Remember, air can't suck up anything, but it can push. Did you change the push of air on the top or the bottom of the paper?

Okay, enough questions! Here's what was going on: In both the balloon and paper activities, air lost pressure and stopped pushing as hard. This happened because you blew the air, and it had to "squeeze" between or around the objects. As it "squeezed" through, it sped up, lost pressure, and stopped pushing as hard.