When Arthu experiments...

Arthu dreads when it comes to Biology class. One day, each student was asked to present an experiment by their teacher.

While waiting for his turn, Arthu experiments on his cockroach. First he cut off one leg and told it to 'walk! walk!' The cockroach walked. Then he cut off its second leg and told the same. The cockroach again walked.

Then Arthu cut off the third leg and did the same. Finally, he cut off its fourth leg and ordered it walk! But the cockroach didn't walk.

Suddenly Muthu said loudly, 'I found it. If we cut a cockroach's four legs, it becomes deaf!'

The whole class ?????