This is the longest English word that I know.
I learned it from a teacher when I was a junior high school student.
I thought, "How can you remember the spelling of the word? You are a genius!"
Much later on, I found it is actually composed of several parts:
It's a lot of fun to introduce this word to students in class. They say, "What's that? Is it a sentence?"

Anyway, do you know any longer words than this?