How do you make the exploding potion for harry potter?

I'm not sure that you even need an exploding potion to get into umbridge's office; normally, doors and gates, etc., are opened by pressing one of those green switches that are around the place (just tap them with you stylus to turn them on. Or you can use one of those decoy detonators, if you're seperated from the switch by a fence; the detonators can go through a small break in the fence). So basically , if you can't open the door to umbridge's office, it means that you have to look around all of the rooms that surround it first, to get all of the tasks done/ press all of the switches that you need to to get into the office. Also, the trick with making potions in general is, when one of those hovering flasks appears that you have to fill to the top with grayish blue to know when to stop pouring or whatever, only fill up to the yellow marker on the side of the flask. Not the clearest explaination, but hey... :L

I am on the same part but it won't let me open the doors?????