I remember reading that water is generated as a byproduct of the hydrogen fuel cells on board. And then the water vapour is also recycled and repurified back into the system. So it's a closed water system powered by solar energy inputs.

Of course, the machinery necessary for this loop costs x billions, but the particular molecules of water used in this experiment probably came out of the internal water cycle and didn't cost to get the water there in the first place.

Tissues can be hanged somewhere on a clip, water evaporates, is picked up by dehumidifiers of the spaceship, goes back to the water system and purified back into drinking water.