Negotiating with a 4 year old

On Saturday Geary and I took the kids to Half Price Books. I found a Robert Munsch book that was not yet part of our collection (Mud Puddle) and excitedly showed Noah. "Look, Noah! Do you want to get this book?" Noah barely glanced at the cover as he replied, "Nah...how about this one?" and held up a cheap Dora the Explorer book. I tried to convince him, "How about I read this one to you and you can see if you like it..." He considered this for a moment and then said, "Okay. And when I don't like it we can buy this one." I got the hint...but I still bought Mud Puddle.