Well you can't weigh water in water, dumbfuck.
No, I can. You may not be able to. So let me educate you.
Get a bucket, and tie a rope to it.
Weigh them together, this is X.
Fill the bucket with water.
Dip the bucket in a tub filled with water such that the tub water reaches to the lip of the bucket without running over.
Hang the rope on a spring scale.
Read the ... oh. I see what you mean. Read the scale... you don't read do you?
AHA! Get someone to read the scale.
Subtract X from the reading.
Write '0' down on a peice of paper and kick yourself in the face.