I agree do not teach them biscuit! K and his 2 cousins all know it and they ask for it alot and they sign to each other like "do you want one?" then we have 3 toddlers all asking for biscuits. There is a biscuit tin at playgroup and K will see it come out from across the room and start signing to me from the top of the slide.
The speach therapist was at playgroup yesterday and I was worried that K's saying words in the wrong order (e.g. car, get, me instead of me get the car) might be from the signing, but she said that they only worry about that at 3yo so at 2yo it is perfectly normal. Phew.