What it shows:
The speed with which you tug on a toilet roll determines whether a sheet breaks off, or the roll simply unravels.
How it works:
The force applied to the junction between the sheets of a toilet roll is proportional to the rate of change of momentum of your hand as you tug at the end. Thus a sharp tug (large ∆p) is sufficient to surpass the breaking stress of the perforated junction. A lesser tug however, below the breaking stress, will apply a torque to the roll itself; the ensuing rotation unravels the roll.
figure 1. Toilet roll user's guide
Setting it up:
One need not get too fancy. Any suitable combination of lab bench clamps and rods to hold the toilet paper roll, will do.
A good physics experiment to set as a practical homework exercise. No theory has as yet been developed to explain how to wrap the paper back on the roll neatly.