These are some hard and fast rules I've learned in the four years since the twins were born:
--You can sit them on the potty, but you can't make them pee.
--The maximum buildup of potential energy for a toy box is about six hours (time slightly extended in the absence of sunlight.) If a toy box is not relieved of it contents within this time, it will spontaneously spew toys around the room.
--No matter how many times they say they want broccoli, they don't really mean it.
--The more prepared you try to be, the less cooperative they will be.
--If they are bouncing off the walls crazy, when you send them outside, they will stand quietly by the fence or next to you until you herd them back inside, where they will be crazy once again.
--Toys are more interesting if you are picking them up or packing them away.
I'm sure there are more.