Knowledge: Safety pins became a thing in 1849. Blue jeans became a thing in the late 1800s. The first electric guitar was invented in the early 1930s.

Conversation:
I would love to learn how to play the guitar.
Oh yeah? You mean the fretted musical instrument that has six strings?
Yes. I love the sound of it.
That is nice. Guitars can be divided into two broad categories, acoustic and electric guitars.
When did the electric guitars become a thing?

Response: The first electric guitar was developed in the early 1930s