"Somebody taking up a new strength-training program usually experiences a phenomenon called DOMS - Delayed Onset Muscle Soreness. This soreness is caused by a variety of factors but primarily is the result of microscopic tears in your muscle tissue caused by your work-out. These tears attract inflammation to the area to help protect and heal the muscle fibers. This process requires extra water. So your body will retain water as necessary until the muscles are repaired. This can result in a 3-4 lb weight gain, which can be discouraging to people just starting out."