Read about the research Some scientists learn about biological processes by observing normal organisms, while others compare broken organisms with normal ones. You can do this, too. For instance, you could figure out how a grasshopper hops by watching a normal one hop around. You might notice that it has big hind legs for an insect of its size that kick out when you poke it, apparently causing the bug to spring forth.
Hind legs are important for a grasshopper to hop.
Although these observations make sense, by themselves they do not prove that the hind legs make the grasshopper hop. Perhaps it is really the four legs in front that are most important. Your hypothesis is a good one though, because it allows you to make a prediction that can be tested. A prediction is an IF-THEN statement. IF hind legs are important for hopping, THEN a grasshopper that cannot hop should have something wrong with its back legs.