[Eugenia Gabrielov]

I agree that encouraging responses to that statement are a great way to get an intimidated student interested in the program.

When confronted with such a reaction to your robotics team, it is crucial to remember that this is not just another set of hands in the shop you are training and it is not just another name to add to your roster. This is a person, an individual who holds a great deal of potential and brings with them a wealth of both talent and experience.

The reason I offer this warning is that a few solutions mentioned so far in this thread allude to the future - "you will get better. You will do great. You will be an excellent member of the team." It is very possible and likely that this student's experiences contain discouragement, possibly even ridicule, with regard to academic performance and activities. They are claiming "I am not smart enough" because they might think they are incapable of learning this material.

Give the future to them, not later, but right now.

Why wait? Really take a moment of your time to show them - teach them - an aspect of your robot. Another student could do it as well, and build a bond that will last when they join the team. Let them take the controls in their hands, or feel the drive train and see how the wheels move, and the role each motor plays. Teach them something small to demonstrate your integrity when you say that you will prove to them that they are smart enough.