the base chemical in pot, in trayvon martin's system. it doesn't mean he smoked that day. >> i love this issue. it finds its analogy in dui law. marijuana is a unique substance. when you test positive for thc, it's unlike other drugs in that it doesn't necessarily men mean you're intoxicated at that point if time. if you have alcohol in your system, you're intoxicated and shouldn't be driving. i still think this evidence should come in because it goes to a state of mind at the time this happened. that being said, i do see the prosecution's position on this particular point. >> and dr. kobilinsky, the level of thc is very, very low. >> that's true. in fact, in a regular smoker of cab cannabis, you would expect a background level of 1.5.