Those blue boxes all over the place, I always wondered what they were for

I was at the post office waiting my turn at the kiosk. The group in front of me consisted of three teenage girls. After purchasing their postage and affixing it to their letter, they looked around, confused.

“What’s the problem?” I asked.

“What do I do now?” ask the girl holding the letter.

“Um, you put it in a post box,” I answered.

The girls looked around.

“Where’s a post box?”

You might recall that we are standing in the post office.

I point out a few.

“Oh, the blue things,” she said, as if she had seen those blue things all over the place but had not until now realized what they were for.

I’m old.

The world’s first adhesive postage stamp was issued on this date in 1840.