You run to the water faucet and splash some water on yourself. It makes you feel better for a moment. You get a bucket and fill it with water, then grab the package of dust and note, and take them with you. You fling the front door open and look around.
Across the street is your neighbor Natalie, a girl about your own age. "Natalie! Something's wrong!" you yell. She runs over and you show her the note. You feel yourself drying up and you splash the water on yourself. It delays the dehydration a bit, but only a bit.
Natalie reads the note. "What a weird thing," she says.
You show her the package and she reaches for it. "No!" you tell her. "It's magic... or scientific, or something. I need your help to rehydrate me before this Collector comes."
"Well, I don't believe it unless I can see it," she says. She grabs at your arm and knocks some of the powder out. Some lands on her face and she breathes it in. Steam begins to come from her skin too.
"No!" you yell. "Now you've done it. You're dehydrating. Now do you believe me? I don't see anyone else around to help us. All we can do is get inside bags like the note says."
Natalie panics, but she has no choice but to do that. You get two large clean garbage bags from the kitchen and step into them. Soon you find you can't feel your feet. You feel yourself getting shorter, and you look down and see your feet have already collapsed into powder at the bottom of the bag. Natalie's face is starting to flake away, and neither of you can do anything more than wait helplessly.
A half hour later the Collector comes by, enters your open door, and picks up the two bags of powder. You're much lighter without all the water in the human body. He takes them to his lab and considers rehydrating them. Then, seeing the chance to do an experiment he was planning, he transfers a scoop of powder from you to Natalie, and another from Natalie to you. After that he adds water, reconstituting you--mostly.