We drove past a no parking sign on the street the other day. Russell asked what the red P sign was, and I explained that P was for parking and that the sign meant "no parking". He thought a bit and then added, "And signs with green P's mean that parking is OK." I replied, "Yes, that's right; that's very observant of you."
Then he elaborated further, "And signs with yellow P's mean, 'park faster.'"