Madagascan gecko (Phelsuma madagascariensis) upside-down on a sheet of glass. This gecko feeds on insects and occasionally fruit. Like other geckos, the Madagascan gecko's feet are adapted to cling to steep surfaces. The force that holds the gecko to apparently smooth, steep surfaces is known as van der Waals' force. This is an attractive force caused by the differing charges across a molecule, which cause the two surfaces to adhere to one another.