I give you two bags and four balls. Three of the balls are red, the last one blue. You are asked to put two balls in each bag. The bags are then taken, shaken, and stirred, and put in front of you.
I ask you to pull out a red ball. What is the likeliness of that happening?
After you pull out a red ball, i ask you to pull out a blue one. (At this point you may switch bags.) Which bag do you try to pull the blue from, and what are the chances?
Nothing special, just a simple exercise.