When all the boys are sitting around at the beginning of the movie (before they get pizza) there is smoke in the room, as if someone is smoking. But no one is, and there are no candles or anything that makes the smoke.
The smoke is actually steam coming from the hot water in the sink. They either used real boiling water or they used a gas machine to create the affect that it was real boiling water.
No kidding -- watch Elliot when he's doing dishes later in the flick and he gazes out the window -- BILLOWS and BILLOWS of steam rise out of that sink!
Does the movie take place in San Diego?...'cause I hear they get a lot of smog down there (chuckle)...2nd thought is that when Elliott goes to the shed it's quite foggy, perhaps what that house needs (instead of a drunk alien, a cat woman with a deafening squeal or F.B.I spacemen) is a good dehumidifier!