The simplest explanation I've heard that has helped it click for people is "You can choose Door A, or both Doors B and C". Clearly choosing both B and C is a 2/

The simplest explanation I've heard that has helped it click for people is "You can choose Door A, or both Doors B and C". Clearly choosing both B and C is a 2/3 probability. Then you guide them to understand how the host revealing one of B or C doesn't detract from the fact that you are still choosing both B and C to begin with. And finally establishing that switching is equivalent to choosing B and C instead of A.