Andre the giant and thumbelina are standing in the middle of an ice-skating rink facing each other. andre the giant pushes thumbelina and she goes sliding, but andre also slides a bit backwards. why did this happen?

This is because of Newton's Third law of motion. The law states that: "For ever action, there is an equal and opposite reaction". When Andre the Giant pushes Thumbelina, he has to push Thumbelina using his arms to propel her forward. However, this pushing motion also has an opposite force, which pushes Andre the Giant off to the other direction.