How much filth can I accumulate in one day?

How come when I step on the scale right before a shower, I always weigh more than I do immediately after the shower? I can be totally nude and do nothing except shower, and I'll find that I lost a pound afterwards, even though my hair is now wet. Do I really have that much dirt, sweat and grime that gets washed away? On a related note, I find that I might be obsessively weighing myself a little too often...