[For a point of reference in this story, Jonah calls Nathan's parents "Oma" and "Opa". Knowing that, you may now proceed.]
Jonah asked me, "Why do Oma and Opa do this?" and he touched the tip of his finger to his tongue.
I said, "Waaahhh...??!?" so he showed me. He picked up a book and when he turned the page, he licked his finger first. "Oh!" I said "That's to make their fingers a little sticky before they turn the page.
"We should start doing that!" he said.
"Oh, I dunno... I don't usually need to, so I don't think of it." I said.
"Oh. So, maybe when we're older?" he asked.
"Um, are you saying that only 'old people' do that?"
Jonah laughed, "Opa's not old!" he said.
"Oh, I see. That's good." I replied.
(But I'm pretty sure she must be the youngest "old" person I've ever met!)