When atoms are deposited slowly onto a flat surface, the first one undergoes a random walk on that surface. Eventually a second atom is deposited; in all likelihood it will eventually meet the first atom. Once the two atoms meet they may bond to form a particle with a higher mass and a lower random walk velocity. Because the bonded particles are now more stable and less mobile than before, they are called an "island." Subsequent atoms deposited on the substrate eventually meet and bond with the island, further increasing its size and stability. Eventually the island can grow to fill the entire substrate with a single large grain.