Squashing and Stretching

Grab a lump of playdough and squeeze it. Pull a rubber band. Bend a bendy straw. Every time, you are using a force — a push or a pull — to change the thing's shape.

A ball of dough starts round. Squeeze it and now it's flat. It's the same dough — you didn't add any or take any away — but you gave it a brand new shape with your hands. Changing shape is one of the handy things a force can do. All day long you are quietly changing the shape of things: your socks, your sandwich, your toothpaste, the sofa cushion you flop onto.

Four ways to change a shape

There are four favourite ways to push or pull a thing into a new shape. You have done all four already — probably before breakfast.

Squash — press it and make it smaller or flatter. You squash a marshmallow between your fingers, a cushion when you sit down, a sponge in the bath, sand into a bucket.
Stretch — pull the ends and make it longer. You stretch a rubber band, chewing gum pulled out of your mouth, the waistband of your trousers, a piece of stretchy pizza cheese.
Bend — push it so a straight thing becomes curved. You bend a ruler, a pipe cleaner, a bendy straw, a floppy carrot, the pages of a book.
Twist — turn the two ends opposite ways. You twist a wet towel to wring the water out, the lid off a jar, a balloon into a sausage dog, a piece of liquorice.

Squash, stretch, bend, twist — four little words for four ways your force can hand a thing a new shape.

Springy things spring back

Here's the really interesting bit. When you stop pushing or pulling, some things spring straight back to how they were before, all by themselves. We call those things elastic, or just springy.

Stretch a rubber band and let go — snap, it's short again. Squash a sponge in your fist and open your hand — it puffs back up. Push a spring down and lift your finger — it pops back. Jump off a trampoline and the mat flattens, then flings itself flat-and-bouncy again. Even your own skin is springy: pinch the back of your hand gently and watch it flatten back out. Elastic things really want to be their normal shape, and the moment you let go, back they go.

When you land on a trampoline, your weight stretches the springs round the edge and pushes the bouncy mat down into a dip. But springs are elastic — they really want to snap back to their old shape. So they store up all that stretch like a squeezed-up spring waiting to go, and the moment they can, they pull back hard and fling you into the air. A pogo stick does the very same trick with one fat spring inside: push down, it squashes and stores the squash, then it shoves you back up with a boing. A bounce is really just stretching and springing back, over and over!

Some things stay changed

Other things are not springy at all. Change their shape and they just keep the new shape — they don't bounce back one bit.

Squash a ball of clay flat and it stays a flat pancake. Roll playdough into a snake and it stays a snake. Press a shape into dough with a cutter and the shape stays, ready to bake into a biscuit. Bend a soft metal paperclip open and it stays bent — it won't close itself again. Scrunch up a piece of paper or squash a can and it stays scrunched. These things don't tidy themselves back up. That's exactly why we use clay and dough to make things — you push them into a shape and they hold it for you.

Have a go

Time to try it yourself. Drag the pull slider and the band on the left stretches longer and longer — the harder you pull, the longer it gets. Then look at the band on the right: that shows what happens after you let go.

Now switch the material between springs back and stays squashed and watch the right-hand band change its mind! Springy? It jumps right back. Stays changed? It keeps its stretched-out shape.

Pull too hard and it snaps

Springy does not mean it can never break. Every material can only be stretched or squashed so far. Pull a rubber band a little and it springs back — but pull it far, far too hard and… SNAP! It breaks in two and can't spring back at all. Bend a plastic ruler a little and it springs straight — bend it right over and it cracks. Even elastic things have a limit.

A squash or a stretch changes the shape of a thing — not how much stuff it is made of. Squashed dough is the same amount of dough, just a different shape.
Elastic (springy) things spring back to normal when you let go — but only if you don't push your luck. Pull one too far and it stays stretched, or it snaps.
Clay does not spring back. Clay, dough, playdough and a bent paperclip keep whatever new shape you give them.

Hook a rubber band over your thumb and pull it back with your other hand. You are stretching it — and, just like the trampoline springs, it stores up all that stretch, ready and waiting. Let go, and it snaps back so fast it flings a little paper ball right across the room! A catapult and a bow-and-arrow work the very same way: you stretch the stretchy part, it stores the stretch, and when you release, it springs back and shoots. The trick is always the same — stretch it, store it, then let it spring back with a twang. (Careful where you aim, though!)