look at the questions ansewers above you.

I know now!! I'm waaaay past it. Ok, when you first enter the celler, go as far as you can right then go down, then left, and then go down the grey stairs. Then, see the big box? with the flipendo sign on it? Go infront of it, and say flipendo. Then, climb back up the stairs, and go left, and walk along that edge, goto the bottem of it, and then walk left. Umm.. My minds going blank. Hmm.. You need to walk aways, until you reach a small rat and tipped over bottle. (it's the tonic) The rat will eat some of the tonic, and poof. He'll be big, and you need to defeat him. Then go back up to the main part of the leaky cauldron, and find ron. ^^ Hope this helps