ok judging by what ur saying to me it seems like ur mom is extremly worried about the things u do / the group of people u hang around with..

"she is afraid of it being mixed with anything harmfull. She thinks that somebody is going to put rat poison in it, or add somthing to it so it looks like your buying more then your getting."

if shes thinking that hard i think its best to just wait till you are caught. if u tell her straight out most likely she will try anything to stop u from smoking weed right away, but if u wait till she finds out then u'll get to smoke more! or maybe she'll never find out......

u can try explaining to ur parents that marijuana isnt as bad as it sounds like, but most likely ur parents wouldnt want u smoking anything period.