When you apply a force on a material, such as pulling, pushing, or twisting, that material can either withstand the force and remain unchanged, or it can change in some way, such as stretching, crumpling, or bending. After the force on a material is removed, some materials go back to their original shape, like a stretched rubber band returning to its original size. These materials are known as elastic materials. Some materials, however, don't return to their original shape (or even fall apart), and we usually say that these materials have broken.

Whether a material breaks usually depends on how you apply force to it. For example, if you take opposite ends of a large piece of paper and pull your hands apart, the paper is surprisingly strong--you can pull pretty hard without it breaking. However, it's very easy to break the paper in a different way, by ripping it!

In the case of a water balloon, even though the balloon is full of water and stretched out, the balloon is able to withstand that stretching without breaking, and if you take the water out, the balloon returns to its original size. If you poke that full water balloon with a needle, now instead of stretching the balloon material evenly, you have created a place on the balloon where a ripping-type force can occur, and the balloon rips apart--it pops!

Balloons are able to stretch without breaking because of the material they are made out of. Balloons are usually made out of materials called elastomers, which is a word that combines two words: elastic and polymer. Polymers are long, chain-like molecules that tangle together and form materials with interesting properties. For example, rubber is a type of naturally occurring polymer, and nylon is a type of man-made polymer. One property that polymer materials can have is elasticity. The rubber bands mentioned earlier are polymer materials with this property, and so are balloons, which are usually made of rubber or latex polymers.

This elasticity is why balloons can withstand being stretched without breaking … at least until you poke them with a needle!

The strength of the material from which the balloon is made determines how difficult it is to pop. Filling a balloon with anything, whether water, air, or some other substance, will stretch the balloon material and subject it to tensile (pulling) stresses. Once the stress exceeds the strength of the material, the balloon will pop.

In a filled water balloon that is stationary (not moving), the balloon is kept from popping by pressure from the atmosphere. Contrary to what we might think as "intuitive", atmospheric pressure does not just act downward on objects; it acts in all directions - down, up, from the sides and everywhere in between. This force from all sides exerted by air molecules on the balloon prevents the water in the balloon from breaking the balloon itself.

Rubber is elastic. Think about rubber bands, that stretch. Balloons are made of the same stuff.