Many different fluids will resist mixing when they come together. The surfaces of these different materials "push" against each other to stay apart. For example, steel is heavier than water, yet you can float steel wool on water because their surfaces push away from each other, and steel wool has so much surface that this push is stronger than gravity!

You've probably seen this effect before. Have you ever watched water bugs skip across the top of a pond? They can do that because of the surface tension between their feet and the water.

Detergents reduce this surface tension. With detergents, dirt and oil will mix with water. That's how we clean everything from dirty clothes to dinner dishes.
 
 
Cool Stuff You Can Do

Get an adult to help you do this, and plan to do it somewhere that can get a little wet.

The Floating Needle — Get a clean mixing bowl and rinse it with water a couple of times, then fill it half way with ordinary tap water. Tear off a piece of tissue or toilet paper and place a small sewing needle on the tissue. Now gently lay the tissue on top of the water (like a raft for the needle). The tissue will eventually soak up and sink; and if you are careful not to bump it, the needle will float. You can carefully poke the tissue with a toothpick to get it to sink faster. Watch the needle for a few minutes. What direction does it point? Most of the time it will gradually swing around until it points north.

Drops on a Penny — Lay a paper towel on your table or countertop. Place a penny on the paper towel. Using an eyedropper, carefully add drops of water to the top surface of the penny. Notice how the water curves up and forms a dome on top of the penny. The surface tension between water and air makes the water bulge up on the penny. Count how many drops you can get on top of the penny before the water spills off. Now try it again, but this time use water that has a squirt of dish detergent in it. Be sure to use the stuff for washing dishes in the sink (Joy® or Dawn® or some other hand dishwashing liquid). Don't try this with the stuff you put in your dishwasher (like CascadeÒ or SunlightÒ). How many drops of the detergent / water mixture could you get on the same penny? Chances are, you couldn't get nearly as many drops to stay on the penny, because of the lower surface tension.

Swimming Pepper — Get a mixing bowl full of clean water. If you use the same bowl that you floated the needle in, be sure to rinse it 2 or 3 times to get all of the detergent out. It really does take 2-3 rinses because detergents are very effective at reducing surface tension. Even the little bit left over when you dump out the soapy water is enough to mess this up! Now grab a pepper shaker and sprinkle pepper over the water. Put a drop of dish detergent (the kind you use in a sink to wash dishes) on your finger and then dip your finger into the center of the bowl. Watch the pepper flakes speed to the side of the bowl.

The surface tension of water is like a skin. When detergent is added to water, that skin rips. The rip moves like a wave across the surface and it shoves the pepper ahead of it. If you want to repeat this, you'll have to rinse the bowl out several times to get all the detergent out. To have some fun with someone, put some detergent on your finger first, and then invite them to look at your magic pepper. Get them to try putting their finger in the bowl to see what happens. Then you put your finger in the bowl and watch the pepper scatter. If you're clever you can rub your finger over the top of the detergent bottle cap and get enough on the tip of your finger to make this work. They'll never see that amount of detergent and you can baffle them for a long time while they insist that you do it over and over without ever figuring out why your finger does it and theirs doesn't. Just change the water every time and find a way to rub your finger over the detergent bottle cap when they're not looking, and you'll keep them guessing for quite awhile!