The gas laws (Boyle's, Charles' and Avogadro's, and the Ideal Gas Law) have a mathematical basis. Real gases do not obey these laws exactly. For example, in a real gas two molecules upon collision can stick together for a while, in which time they behave as one molecule. An ideal gas, or perfect gas, only exists in the imagination. Its molecules are volumeless points in space, have mass and velocity, do not attract or repel each other or the walls of their vessel, collide with each other and with the walls of the vessel perfectly elastically without loss of total energy. A real gas approximates to ideal behaviour at high temperatures and low pressures. The gas laws are mostly adequate for dealing with real gases. The van der Waal's equation is an example of an equation derived to give better approximations of the behaviour of real gases than the ideal gas law.