I put my hand over my notebook. A protective instinct. “No reason. I just find it fascinating. Don’t you?”

Dad nods quickly. “Definitely.”

I gather the nerve to ask the question that’s burning inside of me. “Do you think it’s possible? Time travel?”

Dad tilts his head back and forth. He sometimes becomes a bobblehead when he’s deep in thought. “Yes. Theoretically.”

“A scientist at MIT said he thinks it’s likely a human has already done it.”

Dad gives me a wry smile. “I see you’ve done your homework.”

I keep my palm pressed against my notebook and shrug. “I’ve read some articles.”

“You know . . .” Dad trails off.

“What?”

He has a faraway look in his eyes. “Your mother once said something rather poetic to me about time.”

Your mother. The words make me sit up straight in my chair. “Mom was into poetry?”

“No,” Dad says, laughing again. He runs his hand through his dark curls, which are beginning to gray at the edges. “She was of a similar opinion to you on the subject.”

I squirm. I’m not sure how I’m supposed to feel about that.

“We were discussing the shape of time,” Dad says.

“The shape of time?”

“Yeah. The idea that time is circular. Or at least not the straight line that we often conceive of it to be.” He makes a circular motion with his finger.

“And Mom didn’t think time was circular?”

“Well,” he says, and his eyes get that distant look again. As I watch his face, I wonder if he’s time-traveling right in front of me. If he’s going back to his college days, when he first met my mom. “We were talking about time travel and your mother said, ‘I think the answer lies in figuring out the shape of time. And the shape of time is probably like the shape of thunder. We think it’s impossible to map, but that’s because we haven’t pushed our brains to think in that way.’”

Dad claps his hands in front of him. He’s smiling, but not at me. Catching himself, he shakes his head and gives me a bashful look. “Anyway, I thought that was a neat thing to say. It’s stuck with me. Your mother had a brilliant mind. She saw the world differently than other people.” He tips his head toward me. “Much like you, kiddo.”

“The shape of thunder,” I repeat. I reach for my notebook, and I scribble the quote. I put asterisks around it, which is what I do whenever I’m taking notes in class and my teacher emphasizes a fact so much that it’s obvious it will be on the next quiz.

There’s a faint ringing in my ears. Thunder. Perhaps this is the thing I’ve been missing.

The shape of thunder. What a strange phrase. It doesn’t quite make sense. I would say it’s an oxymoron, but it feels even bigger than that. It’s more than contradictory; it’s impossible. A thing that doesn’t actually exist, but possibly could. An impossible thing that could actually be possible.