Where does it go?

I have spotted something else about my human: something shocking.

I don't know how I never noticed it before.

When I do my dirt, I do it in my outside territory, then hide it. I'm not proud of it, and I like to keep the area clean, so I bury it.

If I do my dirt indoors, I do it in a box full of little stones... then bury it under the stones. It then disappears. I've seen my human put my dirt into a bag, with a scoopy thing, so that must be what happens - he puts it somewhere else.

But... my human did his dirt while I was still nearby. He did it in this big white thing that he always sits on.
When he was finished, he pushed some button, then got up.

When he'd gone, I walked over to the big white thing, jumped up, and looked into it... and his dirt was gone!
There was nothing there - just a puddle of water. Where did he put it? You can't bury things in water... and there was nothing there anyway.

I'm so confused. I looked at my human with suspicion all day.
What does he do with his dirt?