It's plenty strong enough for an average adult, but is not the most durable piece of furniture you'll ever have. I originally built this in 2004. It still survives, but is a little busted around the enges and saggy in the middle.
As in my other work here, I am interested in making things 1) for free, 2) that are recycled, 3) make imaginative use of material limitations, and 4) are fun to have around. I tried to take advantage of the graphics on the boxes in the design. Perfect "nomadic" furniture for the college student, frequent mover, or cheapskate.
Step 1: Cut
Lay three tubes out flat, graphic-side down. Grab a ruler, some wood glue, and a sharp x-acto knife or box cutter. Generally, the stool should be about sixteen inches high, so you can find the center of the box, length-ways, then measure sixteen inches to each side to start your cuts. Cut two boxes as shown. These will be the legs of the stool.