Recently, I read this:
'Those who can, do. Those who can't, teach.'

But, I cannot agree with it. Should it be not:
"Those who can, do. Those who can't preach."

Most of us use 'teach' and 'preach' probably interchangeably and is the cause for confusion. To set it right probably we should define
Preach: Those who expect their version of the stories to be heard without any questioning.
Teach: Those who accept the version of stories told to them allowing every question in the ongoing process.

Probably the words mentioned in italics is the source of misconception for the statement which we started with.