If you rub the palms of your hands together quickly you can feel this resistance as well as heat - you may even remove tiny pieces of dead skin from the palms of your hands. You can also hear your hands rubbing together. Machine parts rubbing against each other may gradually wear away, make noise, slow down the motion of the parts, and heat the parts. The machine can even break!

Two ways engineers reduce friction between parts are rolling and lubrication. Small steel balls (ball bearings) or cylinders can be placed between two surfaces. Very little friction is created as the balls or cylinders roll against the surface.

Although we often think of lubricants as oily liquids that is not always the case. Water and other liquids are often used as lubricants. The lubricant coats and separates the two surfaces so that their small rough spots do not rub against one another. Lubricants can be thick or even paste-like substances like grease, petroleum jelly, lip balm, or very light oils or non-greasy liquids.

Some lubricants work better with certain types of machinery based on the surface roughness, amount of rubbing, wear, or heat generated. Certain machines can only use certain types of lubricants. Using the wrong lubricant can actually damage the machine or the surface of the parts. When you try the Slime! experiment notice some liquids coat the cubes while others dissolve the cubes.