Gecko lizards use their specialized feet to climb walls. The bottom of their feet have millions of tiny, hair-like structures called setae, which allow them to adhere to surfaces like glass and walls. These setae are so small that they can create molecular forces of attraction, called van der Waals forces, which allow the gecko to stick to surfaces without any glue or suction. The more the gecko presses down on its feet, the stronger the forces become, enabling it to climb up walls and other smooth surfaces.