A working knowledge of surface tension allows you to shove a skewer through a balloon without popping it and a pencil through a plastic bag full of water without spilling. Make your nieces and nephews think you're cool.
Surface tension occurs when a group of molecules are more attached to each other than they are to the molecules around them. This can be because they are a series of interwoven polymers, like plastic, or because they are polarized atoms attracted to each other, like water.
The point is, they feel a force that pulls them together. Balloons are basically a layer of rubber trying to pull itself back into shape against the outward pressure of the air trapped inside it. It's that constant pull back towards itself that allows you to stick a pin, or a knitting needle, through a balloon without popping it.
The important thing is to not stick the pin in fast. For beginners, it's also important not to stick it through the sides of the balloon. At the sides, the balloon's material is often already stretched to capacity and the sharp motion won't give the rubber time to squeeze around the pin. Instead, insert the pin at the tied-off end of the balloon and let it come out at the slightly denser rubber at the tip. Do it slowly and carefully, making sure it doesn't overs-tretch the material around it. As the pin pushes in, it distorts the rubber. If it lets air leak through, forcing the rubber farther apart in a tear, the balloon bursts.
Once the pin is in, the balloon should deflate. The air is straining at the sides of the balloon, but it needs a hole to open in order to get out. The pull of strings of polymer in the balloon is great enough that even though the pin breaches the skin of the balloon, the material around the pin squeezes around the intruder so tightly that the air can't shove its way out. The balloon reseals.
The same thing happens with plastic bags filled with water. If a pencil, or other smooth, regular object is inserted into the bag, the plastic will squeeze tight enough around the object that it can resist the pressure from water within.
You can see surface tension working in water droplets. Water droplets don't have an obvious skin, but they are held together by a membrane of surface tension. Individual drops on horizontal surfaces build up over time, getting taller and fatter, until they look like balloons. A fast touch will make them collapse, while a thin, slow-moving object inserted into the drop won't splatter it, letting it keep its bead-like appearance.