Time, Would You Knock It Off?

A few months ago, my son passed up his Grandma Lyon in height. Naturally, being as he's a teenage boy and getting taller than others is one of his missions in life, he was thrilled.
"I wonder if I'm taller than Grandma Luthy," he said.
"You're not," I assured him.
At the time, she was out of the country, remember, so he couldn't check.
"How can you know for sure?" he asked.
It didn't take much to burst his balloon. "Because I'm shorter than my mom, and you're still shorter than I am."
"Oh...