The simple answer to your question would be buoyancy. Anything that floats on water (I'm assuming you mean water) has more buoyancy than water, by definition. A lot of different things affect buoyancy, but one of these things is air content. Wood and cork float for the same reason that life jackets and Styrofoam float: these materials have a lot of air in them, which makes them extremely buoyant.

If you've ever looked at cork under the microscope, you've probably noticed it has a lot of holes. These holes trap air. It's the same thing with wood and foam. You can think of these materials as made up of thousands of tiny air bubbles. Why ice floats is a little more tricky. This is basically the same reason why boats float.

An ancient Greek mathematician named Archimedes first came up with this principle while sitting in the bath. He noticed that as he lowered himself into the bath, the water level rose. Basically, his body was displacing water. If the object displacing the water (human, boat, ice cube) weighs less than the water it displaces, the object will float. You probably know that ice expands when it freezes. Compared to water, then, ice has more volume for the same mass, and so weighs less than the water it displaces. This is why ice floats. If water did not expand when it froze, then ice would not float.

If you've ever been swimming in the ocean, especially a really salty ocean like the Mediterranean, you've probably noticed that you float easier than in a pool. Can you think why? Also, why do you think a feather floats?

They are less dense than water. Pure water has a density of 1 gram / cubic centimeter, or 1 gram / milliliter. (1 milliliter = 1 cubic centimeter). When water freezes, it expands into a crystal lattice and its density is less than that of liquid water. Cork and wood, too, are less dense than water - although not all woods. Here is an interesting fact for you: did you know that the planet Saturn is less dense than water? It is made of different ices, and its overall density is less than liquid water. So, imagine if you could make a bathtub big enough for the entire planet, it would float!

Wood, cork, and ice float in water because they are less dense than water. What does that mean? Well, you've probably heard the old trick question, "What weighs more: a pound of lead or a pound of feathers?" A pound is a pound, so of course they both weigh equal amounts. But if I asked, "Which takes up more space?" You'd tell me that the feathers do. Scientists would say that feathers are less dense than lead, because they take up more space for the same weight. Now think about dropping a rock in a glass of water full to the very top. Some of the water will spill over, because the rock is now taking up space at the bottom of the glass that used to have water in it. However, if you drop an ice cube in the water it will float. It floats because it weighs less than amount of water it would have to push out of the glass if it sank. Wood, cork, and ice are all less dense than water, and they float; rocks are more dense, so they sink. Thanks for the question.

To explain this fact, I would ask you to observe carefully what happens when you put any object in water. The object starts to sink a little. Then two things can happen. The object stops sinking, it floats or the object sinks completely. What is happening? As you introduce the object into the liquid, some liquid is "displaced", you need to make room for the solid object. The liquid will push the object away with a force equal to the weight of the displaced volume of liquid. This is a law of physics and was discovered by Archemedes in Ancient Greece. Now whether the object sinks or floats depends on how much it weights, compared to the weight of water it displaces. If by displacing some volume of water the object receives a push that equals its own weight, it will float, like cork, ice or some types of wood in water. Other objects even by displacing their whole volume of water, do not get enough push and therefore, they sink.

These substances float (in water) because they are less dense than water. This means that a cubic meter of water has more mass (and thus weighs more on Earth) than a cubic meter of cork, wood, or ice. Think about swimming in a pool. You feel lighter in the pool than when walking around on the ground. The is because when you jumped into the pool you displaced a volume of water that is equal to your volume. So you feel a "buoyancy force" pushing you upwards that is equal to the weight of water you displace. You feel lighter because your "effective weight" in the pool is your actual weight minus the buoyancy force you experience. If you are less dense than water, and thus weigh less that than the water your body displaces, then you will float. If you are completely submerged, the buoyancy force will be larger than your weight and, if you don't work to keep yourself submerged, you will rise until you are only partially under water such that the weight of the water you displace is equal to your weight. Why do you think it is easier to float if you take a big breath?

Think about what happens when something sinks in a glass of water. When it does, the water level rises because the object pushed all the water away. All the water that got pushed out of the way had to go up because it couldn't go anywhere else and had to work against gravity to do it.

All the water that gets pushed up has a weight and the object that you dropped has a weight also. The question you have to ask is: is the object that you are putting into the water heavier or lighter than the amount of water that would get pushed out of the way and up if it sank? If it is heavier, then the weight of the object will cause it to sink. If it is lighter, then the weight of the water will keep the object floating. That is what is called buoyancy.