If you religiously live by the scale, you've probably noticed that your weight can (and will) fluctuate [1] a few pounds. Women's Health [2] gives us five reasons that the number on the scale may be up (or down) a few pounds:

1. You were thirsty! Downing 16 ounces of any fluid can cause an immediate "gain" of 1 pound.
2. You're backed up. Constipation can tilt the scale by as much as 2 pounds if it's been more than a couple of days since your bowels did their thing.
3. You're going on vacation. To Thailand. The changes in air pressure can make you retain water on flights longer than 4 hours, which can show up as an extra 1 to 3 pounds on the scale.
4. You ate the Chicken Kung Pao. Have a salty dinner and you could wake up 2 to 3 pounds heavier — your body retains water to dilute all that sodium.
5. Aunt Flo's on her way. Hormone fluctuations in the 3 days leading up to your period can make you retain up to 5 pounds of water.