reglem

Hi, I'm new in electronics and don't know all the right words and stuff! But... I do need some help in the right direction. I want to build a sensing device to sense when device "A" is close to device "B". Like triggering a switch when device "A" is close to device "B". Must be small and light and running on small batteries. Is this possible??? ??? ??? Any advice will be great!!! I seem to be asking something that is impossible!! :'( :'( There must be a way... there's always a way!! ;D