You have probably heard that if you smoke weed, it stays in your system for up to a month. This happens because the chemical properties of THC make it really fat-friendly (lipophilic), so it happily squeezes itself into fat cells throughout the body. It sits there until the body decides to use or recycle these fat cells, so blood concentrations of THC will rise and fall over a number of days, simply due to daily physiological changes of the body.