G1.5: How high does gravity go, Daddy?
Me: What do you mean? Like, if you threw something that high, it wouldn't fall back down?
G1.5: Yes. Maybe on top of the roof?
Me: That's a really cool question. Would you believe that gravity goes forever? It does get smaller the farther you go, but there's always a little bit.
G1.5: [pause] Then how come the astronauts float?
Me: [trying to contain my excitement at the spectacular question] That is such a great question G. Okay... you remember the bathroom scale at Grandma and Grandpa's? The one you step on every couple of minutes while we're visiting them? What does that measure?
G1.5: How much I weigh.
Me: Awesome. How much you weigh is another way of saying "how hard your body pushes against the ground."
Me: So what would the scale say if we put it in an elevator?
G1.5: The same!
Me: Right. If the elevator isn't moving. But what if the elevator is moving up, pushing you up? What would the scale say?
G1.5: [pause] More?
Me: So it would be like you weighed more than you usually do?
Me: Okay, so what if the elevator is falling? What would the scale say?
G1.5: More. [pause] No! Less!
Me: Yeah! It's like you're pushing on the floor less hard because the floor is moving away from you. That is why the astronauts float. The floor is moving away from them all the time.
Me: So how might we test how high gravity goes?
G1.5: Throw something higher and higher, and see if it doesn't come back.
Me: Nice! So how high do you think gravity goes?
G1.5: [confidently] The roof.
G1.5: Because when I throw something on the roof, it doesn't come back.
Obviously, I'm pretty proud that he asked the question about the astronauts. I think that he really did consider the idea of gravity going forever, compared it to the floating astronaut phenomenon, recognized a discrepancy, and then asked the question. Hard to tell what he was really thinking, but I think he really got that one.
I'm not sure what the effect of any of the rest was. I could walk away feeling pretty proud of the whole thing, but in retrospect, I think I was succumbing to pseudoteaching. I explained and asked leading questions, and he gave me the answers I wanted. I think he got something from it, and was really thinking about the implications as we talked. But his last comment, while freaking clever, suggests to me that he didn't make any lasting changes in his mental models.
If I had been more thoughtful after his first question, I wouldn't have gone right for the exciting conclusion, which I know that most adults don't understand, and would have started with talking about how to test how high gravity goes, and whether there's anything we could measure that would give us more insight.