This animation illustrates how a cancer tumor forms in a bed of healthy cells. The animation goes on to show how the tumor recruits blood vessels and how metastasis occurs. A tumor consists of cells that are dividing at an abnormally high rate, crowding surrounding healthy cells and competing for resources. Tumor growth typically proceeds in 3 dimensions, pushing out from the surrounding normal tissue and growing to become visible to the naked eye. A tumor must recruit blood vessels from the surrounding tissue to grow much bigger than 1 mm across.