The harder they're hit, the stronger they are. They stop bullets and let people walk on custard. They're non-newtonian fluids.
Here's a tip for obsessive lovers, smothering parents, and over-friendly friends; If anyone tells you that if you love something you should set it free, show them a non-newtonian fluid. Hold it lightly, and it will slime its way unpleasantly through your fingers. Mash it quickly with all your strength and it becomes strong and useful. A light hand is not the way to touch a non-newtonian fluid.
Also known as dilatants or shear-thickening fluids (like love, they have many names), non-newtonian fluids are gloppy messes when no strong force is acting on them. They literally need a bowl to keep them contained, making them more structurally unsound than a good flan. When they feel an impact, though, they become solid. The more sudden force is applied, the more solid they become. If you try to stir them hard enough, they break the spoon. No surprise, since when they're in the right composition, they can bear the weight of a human being.
And, of course, there's the non-newtonian fluid that may be stopping bullets pretty soon. The bullets go fast enough that they would cause the liquid to become an impenetrable solid. (Though a slow bullet or blade might go through them – Dune becomes reality.)
Despite their exotic properties, non-newtonian fluids are pretty easy to make. A good amount of cornstarch mixed up with water will do. What's needed is a number of particles suspended in just enough fluid to allow the particles to glide past each other like they're a liquid. The particle mixture needs to be dense enough that, when it feels a shock, the particles jam together and behave like a solid. Three fourths of a cup of cornstarch with one third of a cup of water poured over the top will make a little non-newtonian fluid for you. It's not big enough to walk on, but if you have a stereo and a cookie sheet it could be hours of fun.