How do u use a furnace in Pocket edition mine craft

You have to put whatever it is you want to cook or smelt in the top part of the furnace, and combustible material, such as coal or wood, in the bottom part. I have no idea why this wouldn't work, unless the item you are trying to cook is something that simply can't be cooked (ex: you can bake a potato, but not a pumpkin).

Something else occured to me: are you playing in creative or survival mode? I think in creative mode certain Items, like the chest, don't work and are simply decorative. Maybe that's the case for the furnace? I play survival mode so not sure about this, it's just an idea...