What you Need
What to do
Cut a length of tape measure about 15-20cm long.
Warning - the cut edges will be very very sharp
Cover the cut edges with some kind of fairly tough tape, to make them a bit safer, though still be careful.
Curve the tape around a tight radius of around 6-10mm so it just starts to deform, you can either do this by rolling it around a pencil or I found it easier to just bend it between my hands until you can feel it starting to deform.
Try bending some tape with the inside of the curve and on the outside, and see if it behaves differently.
If you over do it you may need to bend the tape into its original curve a little.
What may happen
You should find that the tape has two stable states rolled and straight. Depending how you rolled it the tape will behave slightly differently.
Why does it happen?
The tape measure starts out with a curve across the tape, which gives the extended tape some depth making it much stiffer than it would be otherwise. This is done by deforming the steel tape in the factory so that the bottom is just slightly longer than the top, making it curve.
When you deform the tape, you do something very similar, but this time bending it in the other direction into a coil.
This gives two stable states, but to get from one to the other without distorting the tape even more it has to go through a flat stage. And this stage is very unstable, (particularly for tapes which have been bent backwards) giving the structure two stable states so it is called bistable.