There is a widely used herbicide (weed killer) called Roundup. Roundup kills any plant that it touches. Scientists have genetically altered soybeans and other crop plants to create "Roundup Ready" strains that are not killed by Roundup. By planting Roundup Ready seeds, a farmer can control weeds by spraying Roundup right over the crop. The crop completely ignores the herbicide, but the weeds are eliminated.