Fluctuate!

HI, I jump back and forth on the scale 3 to 4 pounds regularly; I weigh myself at the same time; in the what I entered the world in, and can't understand why I can go up and down 3 to 4 pounds in a day; I am keeping pretty close to my target every day so its not because I ate 10 slices of pizza or drank only water that day (I do neither) Anybody else find this?