Oobleck is a mixture of cornstarch and water. It is a non-Newtonian fluid: the harder you stir it, the thicker it gets. Hit with a hammer, oobleck hardens enough to shatter. Compare it with a solid and a liquid. A solid, like an ice cube, keeps its shape. A liquid flows and takes the shape of whatever container it is poured into. It thickens when you push on it hard.

Solids, liquids and gases are the three states of matter commonly found on Earth. All three are made of tiny particles such as atoms and molecules. In a solid, the tiny particles are packed tightly together, often in a regular pattern. They vibrate in place but do not move around past one another. In a liquid, the particles are close together. They can slide past one another. In a gas, the particles are far apart. They move around on their own.

Ice floats because it is less dense than liquid water. In most other substances, the solid is denser than the liquid. Water expands when it freezes, so ice takes up more room than the liquid water it came from. Water is densest at about 4 degrees Celsius, or 39 degrees Fahrenheit. Below that, it begins to expand as it cools toward freezing. At normal air pressure, water freezes at 0 degrees Celsius, or 32 degrees Fahrenheit. It boils at 100 degrees Celsius, or 212 degrees Fahrenheit.

Air, the gas around us, is matter. Gases take up space, which is why a balloon inflates when filled. Air has mass, and because of gravity on Earth it also has weight. At sea level, the weight of the air pressing down is about 14.5 to 14.7 pounds on every square inch. Air pressure is lower high in the mountains because there is less air above. For example, Leadville, Colorado, which is 10,200 feet up, has about 68 kPa, against 101.3 kPa at 1 atmosphere.

A phase diagram plots pressure against temperature. It shows which state a substance is in under each condition. Dry ice is frozen carbon dioxide, and it turns straight into a gas at about -78.5 degrees Celsius. Carbon dioxide cannot be a liquid at normal air pressure, so dry ice never melts into liquid in the open air. Squeezing a gas can also turn it into a liquid, if the gas is not too hot. Gases can be turned into liquids by cooling them. Liquids can be turned into solids by cooling them more.

When liquid water is heated enough, it turns into a gas called steam. Evaporation is liquid water changing into water vapor, a gas. It happens when heat energy makes water molecules break free of their liquid. Evaporation takes heat away: when water on your skin evaporates, you feel cold. Condensation is water vapor, a gas, turning back into liquid water. Fog forms when water vapor in the air cools and condenses into tiny drops of liquid water near the ground.

Melting is a solid changing into a liquid. Freezing is a liquid changing into a solid. While ice is melting in water, the mixture stays at 0 degrees Celsius. Some solids skip the liquid stage. Sublimation is a solid turning straight into a gas with no liquid stage. The reverse is called deposition. Snow and ice can sublime, slowly turning into water vapor without melting first. Plasma is a fourth state of matter, after solid, liquid and gas, and the Sun is a ball of plasma.

Words to know

• oobleck: cornstarch mix
• particles: tiny bits
• pressure: pressing weight
• sublimation: solid to gas
• deposition: gas to solid
• plasma: fourth state