I keep seeing things I forgot I forgot.

Like this time we were hiking. We had just gotten back to the car and we were watching the sun go behind our ridge to the west.

HIM: If we hiked for an hour at four miles an hour, how far did we hike?

ME: Duh!

We laughed.

ME: I won’t be in fourth grade forever, Dad.

He nodded.

ME: If we hiked for four years, how far did we hike?

He laughed. I loved when I could make him laugh.

ME: Here’s one. Why do we still see half the sun if the sun is already gone? It’s already below the horizon?

HIM: Yeah, that’s a good one.

ME: It’s light is curving around the earth?

He nodded.

ME: Because of Einstein?

HIM (smiling): I don’t think we can blame Einstein this time.

ME: The earth bends spacetime so light thinks it’s going straight but actually it’s curving?

HIM: The sun does that, for the light of stars behind it, right?

I nodded.

ME: Their light curves around the sun so to us they look farther out from the sun.

He nodded.

HIM: But is earth that big? To bend the light from the sun?

ME: OK no.

HIM: So why is the sun’s light curving around the horizon to us?

ME: Like a magnifying glass?

He nodded.

HIM: Where’s the magnifying glass?

ME: The atmosphere?

He nodded.

We watched the sun disappear.

ME: If you could hike to Saturn and back at the speed of light, or almost the speed of light, would you be younger when you got back?

HIM: Let me think. OK. The one who’s accelerating and decelerating accumulates less clock time. So yes, our age difference would be smaller when I got back.

ME: So you could come back when you’re the same age as me? And be a kid again with me?

HIM (laughing): Slow down. Try again.

ME: You can’t get younger? I can only get old faster than you.

He nodded.

ME: I’d be old and decrepit like you.

We laughed.

HIM: Time is still a one-way road. You can never go back. Not even Einstein can fix that for us.

ME: Like that movie.

HIM: Which one?

ME: The daughter is mad the father left for Saturn or wherever.

HIM: Interstellar?

ME: With all the crying.

HIM: Yeah, that’s the one.

ME (imitating): I’m sorry. I’m sorry. I’m sorry.

We laughed.

ME: Remember they want the father to go on some little side trip out in space and he doesn’t want to? No one understands why. It’s only two weeks. What’s his problem?

He nodded.

ME: But two weeks of acceleration for him is another two years for his daughter on earth. He loses another two years with her. If he ever talks to her again he’ll be like her little brother.

HIM: Little brother? Does he get younger?

ME: OK no. But she could be old as him, like his sister.

He nodded.

ME: Or older than him, like his mother?

He nodded.

ME: So if I send you to Saturn and wait for you here, I get old faster and I could be your mother when you get back?

We laughed.

ME: But we still wouldn’t have any more time to be together?

He shook his head.

HIM: Less. Always less.

ME: But I wouldn’t be as young when you die? I would die closer to you? I wouldn’t have as long after you?

He nodded, still smiling that sad way.

ME: OK, get going. Hurry. Accelerate a lot. I’ll wait here.

We laughed.

ME: Wait, give me the car keys. I’ll drive us home when you get back.

We laughed.

ME: In case you can’t remember where we live….