“You are studying computer science. Can you tell me exactly what is a computer?”
Go ahead, try it. Don't look up anything, don't consult friends, and don't take hours or even minutes to think about it. Imagine being stared at by four or five august members of faculty, under the illusion that their reaction affects whether you get your degree. And then imagine your subsequent panic when Prof. Law says, “By that definition my X is a computer” where X might be a bicycle, a sewing machine, possibly even plumbing.