COCKROACH!!

Once a medical student makes a lot of mess in the classroom and so the lecturer got fed up with him.
He asked the student to go to the medical lab and make some observations on a cockroach.
The student then goes to the lab, takes a cockroach,puts it on the table and says "walk",
so the cockroach walks,
he writes this in his observation book,then he cuts 5 of its legs one after the other and asks it to walk each time
and the poor cock-- walked.and he notes this in his observation book,
now after cutting the sixth leg (the last leg),when he asks the cock-- to walk
it doesn't and so the student writes ,that on cutting the sixth leg of the cockroach it CANNOT HEAR...poor cockroach..