RAY: We're back. You're listening to Car Talk with us, Click and Clack, the Tappet brothers. And we are here to discuss cars, car repair, and the new puzzler. And here to help us, believe it or not, is Martin Gardener's son, Jim, who is going to deliver today's -- this week's, puzzler.

JIM: Yes.

TOM: So, you get all the hate mail Jim. Martin Gardner is the father of puzzlement and Jim is his son, so Jim must be puzzlement.

RAY: And Jim called because I guess, I said Dad had left us and Dad hasn't left us.

TOM: And he has no intention of leaving us either. So, Jim, what is this puzzler?

JIM: There are three on and off switches are on the wall on the first floor of a building. You can tell when they are switched on or off. They have got things that say on and off. One switch controls a bulb in a lamp on the third floor of the building. The other two switches are dummies. They're not connected to anything. You are allowed to toggle the switches as many times as you please, and then you are allowed to walk just once to the third floor to check the light bulb. How can you know then which switch controls the bulb? And that's the Martin Gardner puzzler.

RAY: OK.

TOM: That's very, very good.

RAY: OK. This is a test of your memory. Can you state it again?

JIM: Are you asking me or your brother?

RAY: No, you.

JIM: I've got it written down in front of me.

TOM: Oh, he's got it written down.

JIM: Do it again?

RAY: See, no wonder it was so smooth. Should I do that?

TOM: No. Don't. You'll spoil everything.

TOM: A second time just to make sure that the slower among us have an opportunity to jot down whatever little clues.

RAY: I got lost with the three switches.

TOM: Three switches on the wall. One of them is attached to a light on the third floor and the other two are dummies.

JIM: That's correct.

RAY: OK. Like us.

JIM: Three on and off switches are on the wall on the first floor of the building. And you can tell when they are on or they are off, but only one switch controls a bulb in a lamp on the third floor. The other two switches are dummies. They are not connected to anything. You can toggle the switches on and off as many times as you please, but then you are allowed to walk to the third floor just one time to check the light bulb. How can you know which switch controlled the bulb?

RAY: OK. It is an honor to have spoken to you. Please give my very, very best to your father, and more importantly great, please give us the answer.

JIM: Oh, you mean I have to do that?

TOM: Not now. No, you don't have to.

RAY: No, you could email it to us.

TOM: I think I have the answer.

RAY: I may too, but I have to work out the details.

TOM: I think I have it.

RAY: Jim, it is a pleasure.

JIM: Well, good luck and a pleasure to you two gentlemen.

TOM: Thanks for calling. Bye bye.

[ Car Talk Puzzler ]