According to Chemistry Land, suction cups work by pushing the air out of the inside of the cup, so that the air pressure on the outside of the of the cup holds the cup against the surface. (See References 1) The cup stays in place due to friction. If your child is interested in how suction cups work, there are several experiments your child can perform.
Surface Holding Ability
A basic experiment for understanding how suction cups work involves placing suction cups on several different types of surfaces. The student can test several types of surfaces, including wood, ceramic tile, textured tile (where the surface is not smooth), metal and glass. In order to perform the experiment, the student will need the various types of surfaces, suction cups, string and small weights. The student times how long it takes for the suction cup to release off of the surface after the weights are attached; several will fall off without a great amount of weight, while others will take longer and more weight to remove from the surface. The student can then graph her data and use this data to determine the surface to which suction cups adhere the best.
Artistic students or children can focus less on the physics of suction cups, and more on experiments that allow them to play with suction cups' artistic possibilities. One type of project is to make an installation using suction cups. For this project, the student needs several suction cups, paper, paint or markers, scissors and string. Allow the student free reign to experiment with how suction cups can increase her ability to make art. Focus on concepts of artistic installation or the effects of gravity on beauty. For an advanced project, have the student make a miniature version of an artistic installation using suction cup technology.
The Wonders of Our World website details an interesting experiment in which the student makes a suction cup from a normal plastic cup and an index card. (See Reference 2) In this experiment, a student fills a plastic cup with water. She then wets the rim of the cup and tightly holds the index card to the top of the cup. While holding the card in place, she flips the cup over. When her hand moves off of the index card, the card stays in place. This is a great experiment to use to explain atmospheric pressure. The card stays because the pressure on the outside of the cup is greater than the pressure from inside the cup.