Do you want a brief explanation of what an acorn is?

Behind the Groan

An Oak Tree in a Nutshell

“In a nutshell” means giving a short, simple summary. An acorn is the seed-like nut from which an oak tree can grow, so the future tree is also literally contained inside a nutshell. The request for a brief explanation points toward the familiar expression before the answer makes it physical. Keep the setup conversational and pause after “In a nutshell” so both meanings have room to register.