You in the hands of transparent glass with liquid dark color. For persuasion lower in the glass line and show that it was painted in black color. Then cover the glass with a handkerchief. With the words: "My powder discolor any drinks!" slightly lift the edge of the scarf and pretend that poured into glass powder. If you now remove the handkerchief, the glass will be clean water!
What's the secret? In a glass of water is inserted into the cut out in the shape of a leather liner black (or any other) color. So it seems that in the glass of the drink. Line with one side painted in the same color. Dip it in a glass of clean side to the audience, and then quietly turn the other side and show.
The powder need only to confuse the audience and produce the desired effect. If together with a scarf to pull out of the glass thin liner, the drink will turn into water.