Now, we still don't know exactly which one it is. It can be either this or it can be this. The question is, now what experiment can we actually conduct with this plant to determine if this if the genotype is this or this? Well, if we take this plant with the unknown genotype and we cross it with a plant whose genotype we actually know, then we can figure this problem out. And the only type of plant whose genotype we know for certain is a homozygous recessive. So a short plant, because the only time a plant will be short is if its genotype is actually lowercase T. Lowercase T. And this is what we call a test cross.