These are instructions on how to play as an invisible bear. I've come across this glitch by accident. After the puppy's box has been opened and has stolen three letters, find at least one letter then go back to Kayla's room.
Gretchen will tell Fatty Bear that the puppy stole three of the letters for the happy birthday sign. Don't put up any of the letters yet, exit the room then come back in. Exit the room again then go back in one more time.
While the clown is talking, quickly click on the balloons. If you can't get there quick enough, don't worry. Just click on the veggie head, then try again.
If you've done it right, Fatty Bear will be blowing a balloon while saying his dialog. If the puppy comes into the room and pops the balloon, Fatty Bear will be stuck in the same pose but will still move around. At this point, you can you exit the room and play as an invisible bear. (If the puppy does not come into the room right after he blows a balloon, you'll have to try again.)