I vaguely remember the following conversation from back when I was a PhD student.
Student A: What’s a Bessel function?
Student B (waving his arms about): It’s a wavy thing – goes like this, doesn’t it?
Me: Sounds vaguely familiar – I think we did it in third-year.
Student A: But what IS it?
Me: Something to do with waves on a membrane, isn’t it?
Student A: Does it have a formula?
Me and student B: Don’t know…