My OH got herself a new set of bathroom scales in the sales as our old pair broke (And NOT because I stood on them). She tried them out.
"You can't use them on the carpet they give a false reading" I said
"How come?" she asked
"It's because the scales don't actually measure the weight acting down upon them, but the reaction of the floor acting upwards against the weight"
"Isn't it the same?"
"On the hard kitchen surface yes, but when the carpet is soft it isn't exactly the same, you may appear lighter"
So she went and weighed herself using the scales on the hall carpet. "Ooh I'm half a stone lighter on the carpet"
"See!" I replied.
That's great.....
....
"All I've got to do now is just walk on carpet and I will have lost half a stone"
Gotta love her!