What is "play"?

Jane McGonigal suggests that games have obstacles.  Her famous example is golf: if the point is to put a ball in a hole, the easiest way is to just go and put it there, but the point of golf is to stand far away from the hole and hit the ball with a stick.  Scrabble too: if it's about arranging letters into words, we already have an activity for that, and we call it typing.  But in Scrabble, you have obstacles: you can only play with 7 letters at a time, the word has to connect with a letter of another already on the board, and the letters translate to points which have to be maximised.

This morning, I was in a hurry and wanted to get my 2 year-old's teeth brushed.  She obediently walked most of the way to the bathroom with me, and then suddenly inexplicably decided to lie down on the floor.  She laughed and waited to see what I would do.  She knew that eventually I would laugh too and pick her up and flip her and tickle her and swoop her into the bathroom. 

Now, why would that be fun?  Not only was she deliberately putting obstacles in my way, but getting picked up and tickled is and obstacle to walkingforward.  There's no biological reason for what she did: it didn't help her get food or any other necessity.  It's just  FUN.

There are millions of dollars for anyone who can really define what FUN is.