James, Albus and Lily had the best idea of a gift for grandpa Arthur: they would bring him to the movies! He'd love it, he was always talking about his love for Muggles, and their parents brought them often enough to the Muggle theatre in the next town that they knew their way around it. It would be great!
On the long-awaited day, therefore, Arthur was seated comfortably between his grandsons, a huge bowl of popcorn on his knees in which Lily kept plunging a hand, leaning across her brother to reach it.
When the light in the theatre went down, James gave a pair of dark glasses to his grandfather.
"What are these?" Arthur asked.
"3D glasses. Put them on to see the film in three dimensions."
Arthur looked at the plastic glasses, eyebrows raised.
"Don't we already see in three dimensions?"
"Yeah, I guess," replied James. "But normal movies are in two dimensions, you need the glasses to see that one."
"How can we see something made in three dimensions only in two dimensions?"
James glanced at Albus, who shrugged. Lily wouldn't know the answer, of course: she was only eight. So the boy turned to his grandfather, a huge smile on his face.
"It's Muggle technology, grandpa."
Satisfied, Arthur nodded, slipped the glasses up his nose and leaned back to enjoy the movie, not noticing his three grandchildren collapse in a silent fit of giggles around him.