Let me demonstrate a little. Here's one example of a black box I set up:
(There are 3 other arrangements. Again, this is an activity I adapted from FOSS: Models & Designs.) I use a lot of tape, having learned a long time ago that the impulse to cheat is irresistible. There's a little ball inside, and it runs up against these cardboard or foamboard obstacles. Students have to figure out what's inside the box based on the sound and feel of the moving object. I don't tell them anything about what's inside. They usually focus first on the moving object, and the most common inferences were marble or battery. Battery seems an odd guess, but the obstacles make the object move in strange ways, and a cylindrical shape is a plausible explanation with some of the boxes. At some point during our discussions, a student will object to the battery hypothesis and propose that there are other things in the box (obstacles or whatever) that cause the marble to move in funny ways. Students argue and become quite attached to their ideas, holding on in the face of strong evidence against them - this activity mirrors what happens in the "real" scientific community in many ways!
Keep in mind this module from FOSS is geared toward 5-6th graders, and I can't imagine why. My 8th graders have a very low tolerance for intellectual frustration and had difficulty devising any methodology for figuring out what was inside. Some students sat there repeatedly, incessantly shaking the box near their ears, even a few days into the activity, expecting some magical voice, I suppose, to reveal the contents to them. I modeled a few strategies for how to work the box without announcing that I was modeling a strategy - rolling the moving object around the perimeter, noting where there was an obstacle, mapping it on a piece of paper, etc. I should have announced it. Toward the end I thought of another technique, and had students tape an index card directly to the box and start marking where the obstacles were. That was a big help and I wish I had thought of it earlier.
Oh, what is the twist that I added to the original FOSS idea? I used a steel ball instead of a glass marble. At a certain point I alleviated some frustration by asking student to focus on the moving object. Some had suggested a marble. We discussed what a marble was - a glass sphere - and whether it might be something other than glass. Was there any test we could do to determine if it was glass or not? What else might it be made of - wood, rubber, metal - could we do any kind of test that would tell us if it were any of those materials? Someone suggested if it were metal, it would be attracted to a magnet. So out come the magnets. Now we have not only narrowed down the possible materials that the object could be made of, we have a new tool or technology for manipulating the object and creating a map of the inside of the box. A lot of scientific method here, and a little messy like real science can be. Observations, tentative explanations or hypotheses, testing hypotheses, revising the model, arguing, sharing information, inspiration, using technology to refine the model, and so on.
You can download the lab sheet I made with some notes here (MSWord). This is my original lab, without any modifications. I would certainly revise some of it in light of my experiences, but at the moment I'm focusing on the next unit and cannot revise it now. I definitely guided students more than I wanted to, but I had to balance the process of discovery against the frustration level that sometimes threatened to undermine the process anyway. If I have time I will post a student example later this week.
UPDATE
Here's a nice alternative activity from Exloratorium if you can trust your students not to cheat!
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