Adhesives appear in many forms and many places, including natural and man-made compounds. Adhesives are made to hold things together, and that stickiness comes from chemical bonds and the amount of force required to pull those bonds apart. Most adhesives, including adhesive tape, are visoelastic materials, meaning that they are materials with the characteristics of both a solid and a liquid. When left alone, visoelastic materials act like a solid and stay put. But when pressure is applied, the visoelastic material flows like a liquid to fill in cracks and conform to uneven surfaces. In addition to the ability to conform to surfaces, many adhesives form relatively weak bonds based on van der Waals forces. Van der Waals forces are created by dipoles, the separation of positive and negative charges on opposite ends of a molecule that has a net neutral charge. When the positive dipole of one molecule is attracted to the negative dipole of another molecule, the force holding those molecules together is a van der Waals force. Billions upon billions of molecules experiencing those forces keep adhesive tape or the sticky toe pads of a gecko attached to a surface.