Paper helicopters spin because of the earth’s gravity, lift, and configuration of the rotors. When dropped, the helicopter’s mass experiences gravity, and it naturally falls to the floor which causes paper blades to bend slightly upward due to lift. The lift force of the air pushes on each of the blades equally but in opposite directions, horizontally and vertically. As a result of the horizontal equal, opposite, and offset forces, the helicopter spins around as it descends.