Rockets – Force is the word scientists use to talk about different kinds of pushes and pulls. Different forces create motion (anything that is moving has motion). Flight is a kind of motion that requires force. How do rockets use force to fly? A rocket is powered by expanding gas. In a rocket engine, two different kinds of fuels are heated and mixed together to make a lot of gas, and it expands very quickly. The pressurized gas wants to escape. In a rocket, it's directed out through a nozzle and hits the ground with enough force to push the rocket up into the air!

Rockets work on the principle of thrust, which is part of Newton’s Third Law of Motion. Stated simply, Newton's Third Law of Motion says that 'for every action, there is an equal and opposite reaction.' Use a pair of roller skates and a ball to show how this works. What happens when you're standing still in skates and then throw a ball hard? The force of throwing the ball pushes your skates (and you) in the other direction!

Density – If a piece of wax sinks through oil but floats on water, does this mean its density changes? No, an object's density stays the same, but different liquids have different densities. For example, oil is less dense than water, so it will float on top of water. (Oil and water can't mix together, so oil always stays on top.) Honey is more dense than water, so it will sink through oil and water. The way an object acts in liquids that have different densities can be different. An object that floats in oil might still sink in water, even though the object itself doesn't become more or less dense.

Sometimes density changes with temperature. As you found in the water balloon experiment, warm water is less dense than cold water. The same works with air: warm air is less dense than cold air, which is why warm air rises and cool air sinks! In our experiment, we couldn’t see the warm water rise, instead, we saw the balloon filled with cold water sink.

Make a water bottle rocket and learn about liquid density with water balloons with two easy, outdoor science projects.