Ball And Tube

A puzzling piece that will have them thinking for a long time. You show a small steel ball bearing and a small tube. The size of the ball is barely too big to go inside of the mouth of the tube. You set the ball on top of the tube and concentrate. Slowly, the ball starts to sink down inside the tube. But this is impossible! How can this be happening? Then, slowly the ball starts to rise out of the tube. Everything can be examined afterwards!