That saying, or ones like it, are also attributed to Carl Sagan, G. K. Chesterton, James Oberg, Arthur Hays Sulzberger, Virginia Gildersleeve, Bertrand Russell, Max Radin, ...

This article mentions Walter M. Kotschnig as the author of the earliest known instance of the saying, but notes that even Kotschnig seemed to be quoting something that was already known at the time:

https://www.skeptic.com/insight/open-mind-brains-fall-out-ma...

An appropriately simplistic quote from someone who habitually and sometimes famously demonstrates being stubbornly narrow minded. But then, how else would he not know that brains don’t fall out of—nor are placed in—minds.

I prefer Terry Pratchett's quote:

"The problem with having an open mind is that people will insist on coming along and trying to put things in it."