The Black Box Model is an effective learning tool and can be constructed with minimal cost and effort.

MATERIALS
2 pieces of 8 mm nylon rope, each 70 cm long
1 harness ring with a 40 mm diameter
1 piece of 7.5 cm diameter drainage pipe 35 cm long
2 drainage pipe end caps, 7.5 cm diameter

TOOLS
3/8" drill bit
power drill

PROCEDURE
Step 1: Drill the top holes directly across from one another, each 5 cm from the top (Figure 1). Repeat for the bottom holes, each 5 cm from the bottom.

Step 2: Thread the one length of rope through the top holes and the harness ring (Figure 2).

Step 3: Tie a knot 15 cm from each end of the rope.

Step 4: Thread the other rope through the bottom holes. Again, ensure that the rope passes through the harness ring as indicated (Figure 3). Tie a knot 15 cm from each end of the rope.

Step 5: Secure end caps.

Note: Variations on the design (without a ring for example) will enrich the discussion and work equally well. You may wish to encourage students to build their own versions of the device with bathroom tissue tubes and string, but never reveal how the device is constructed.