Thinnet

Meet Thinnet, a stretchy rubber strip who gets thinner every time somebody pulls her longer.

Stretch almost anything and it narrows in the middle. Squash it and it bulges out. Poisson's ratio ν compares the two effects. It is the sideways strain divided by the lengthwise strain, with a minus sign so the ratio comes out positive for ordinary materials.

Rubber has ν close to 0.5, which means its volume barely changes when it stretches. Steel has ν ≈ 0.3. Cork has ν close to 0, which is why a cork slides easily into a bottle without bulging sideways.

Stretch a steel bar so it grows longer by 0.1 percent. With ν = 0.3 its width shrinks by 0.03 percent.

Some unusual materials, called auxetics, have a negative Poisson's ratio. They get fatter when stretched. They are being used in protective padding, sports gear and medical stents. The ratio is named after the French mathematician Siméon Denis Poisson, who studied elasticity in the 1820s.