Students need to learn this. Making a humorous remark about a problem indicates to the students that the problem is beneath their attention. Humorous remarks can disrupt students who otherwise would make a serious attempt to solve the problem. Learning to persevere and rework failed solutions is important. Giving an immediate answer will deter most of the students from making more than one attempt to work a problem.
Any answer is better than no answer.
No one should be allowed to make a remark about an answer unless they have a better one. Student will occasionally argue that they can learn by listening, and that they do not have to participate directly in the process or discussions in order to learn the methods. They are wrong. This puts the student right back in the memorization method. One must participate directly in order to learn how to solve problems.