Mutually exclusive

[ItsGrimUpNorth]

I don't understand the meaning of this expression. Can anyone explain.

Also, don't let's. I understand the meaning of that expression but as a child, reading it in Enid Blyton stories, I was always struck by how peculiar it is.

If you have two things which are mutually exclusive, then A excludes B and B excludes A, so there is just no way that the two things can occur together. A being true excludes all possibility of B being true, and B being true excludes all possibility of A being true. Does that make sense?