Some materials are called colloid suspensions. They are composed of a framework of one kind of particle, which supports other particles within the substance. Lets look at some materials like this, especially ones which behave like a 'house of cards'. A simple example of this kind of suspension is spaghetti sauce, where the sauce itself has suspended within it all kinds of other particles ... seasonings, bits of vegetables, and meat. But a colloid can have particles suspended in it that are too small to see. For example, whipped cream is composed of air bubbles, that help support little bits of butter fat. You can't see the bubbles or the fat ... the substance appears to be a smooth 'solid'. The air bubble structure of whipped cream is really quite strong. It will support a lot of weight. The air bubbles link to each other and form a structure that supports not only the weight of the bubbles themselves, but the butter fat within, and even things you can pile on top. If this 'foam' could be hardened, it would make a light, strong structure. When done with plastics or metals, you get materials like styrofoam or foamed aluminum. The problem, of course, is that colloidal suspensions are not stable. Whipped cream, Jello (another example of a suspension), and other colloids will break down when they are warmed. Eventually they lose all their structure, becoming liquids. Some colloidal suspensions can also break down if sideways forces are applied to the structure. The particles are supported, but only weakly. Much like a house of cards, they will remain in place only as long as no sideways force is applied. When a sideways force is applied to a house of cards, the house falls apart. In some substances that are colloidal suspensions, when a sideways force is applied, the solid form of the substance becomes more like a liquid, as the particles lose their grip on one another. Substances which are thick like a solid, but which flow like a liquid when a sideways force is applied, are called thixotropic. Can you think of any thixotropic substances? A sideways force is often applied by shaking. Ketchup can be a thick paste, not quite a solid, but certainly thick enough that it won't flow. If you shake the bottle, however, the bonds between the particles in the suspension become weaker, and the substance loses some of its structure. It then behaves more like a liquid, and will pour. The dictionary definition of  thixotropy: "a property of certain gels which liquefy when subjected to vibratory forces like simple shaking, and then solidify again when left standing." Toothpaste is also thixotropic. It is much like a solid when left alone. But when you squeeze it, applying a sideways force through the tube, it flows much like a liquid. The opposite of thixotropic is dilantant. These substances get more viscous (even hard) when you apply a force. A really fascinating example of a dilantant material can be observed by mixing corn starch with water. Corn starch is a white powder which you'll find in your kitchen with the baking supplies ... it's used to thicken gravies, sauces, and other liquids. Mix a small amount in a bowl. For our example here, we also added food colouring, so that the mixture would show up more clearly. You'll immediately notice a problem when you start mixing water with the corn starch powder. The powder immediately turns to a thick, almost solid, paste. Keep adding water (or more powder) and stirring until you have a thick, smooth mixture. As you stir, it will clump like a solid. But when you leave it, it will look wet, and when you pour some out of the bowl, it will pour like very thick paint. Go ahead and pour out a little bit onto the table. The substance feels wet. Slowly push your finger into it, and pull it out. The substance definitely resembles thick paint. But wait ... pick up a piece of the material you poured onto the table. Yes, that's right ... it's a solid piece! Now go back to the bowl and push your finger onto the surface of the 'liquid' quickly, and with a lot of force. Hit it hard with your knuckles. The harder you hit it, the more solid it appears. You will be amazed to discover that the material is now acting just like solid rock. When you shake the bowl, however, you'll see that it's also behaving like a liquid! Corn starch and water make a dilantant suspension. The particles are held loosely. When a small sideways force is applied, by shaking the bowl, or pushing into it slowly with a finger, or allowing gravity to pull it sideways when you pour, the weak bonds between the particles break, and the substance flows just like a liquid. But if you hit the suspension hard with your finger, or with a hammer, it turns solid! Dilantant materials increase their viscosity as shear forces increase. Corn starch is, as its name suggests, a starch, which is a long molecule made from sugars. Starch from various sources has been used for centuries as a stiffener. You may have sprayed it on your clothes after ironing, or added it to a sauce. There are mixtures found in nature which are thixotropic. Some types of clay, for example, are quite solid. Clay contains solid particles and water molecules, held together in a fairly rigid structure. Often houses are built over this material, without problems. However, when an earthquake happens, the thixotropic clay gets shaken, and the structure of its particles breaks down, disappearing like a house of cards. The water molecules are free to move around, and they provide lubrication that turns the clay to a liquid ... and the houses fall down. Ordinary mud will sometimes act this way. If you are careful to bring your feet straight down on the 'hard' surface of the mud, you can walk across it. But it you twist your feet while walking, or press too hard, your boots will sink into the now 'liquefied' mud. Resources