Gravity is a pulling force that pulls everything toward the center of the Earth. It is the reason things fall down when you drop them and the reason you stay on the ground instead of floating away. Every object with mass has gravity, but Earth is so big that its gravity is the one you feel every day.
Drop different objects (a ball, a feather, a book) from the same height and observe that they all fall. Use a simple balance scale to feel the pull of gravity on different masses. Discuss why astronauts float in space stations.
Have you ever wondered why everything falls down instead of up? The answer is gravity. Gravity is an invisible force that pulls objects toward each other. On Earth, gravity pulls everything toward the planet's center. That is why when you jump, you always come back down.
The strength of gravity depends on how much mass an object has. Earth is enormous, so its gravity is strong enough to keep you, your house, and even the ocean stuck to its surface. The Moon is smaller than Earth, so its gravity is weaker — if you could visit the Moon, you would be able to jump much higher than you can here.
Gravity is also the force that keeps the Moon circling Earth and Earth circling the Sun. Without gravity, the Moon would fly off in a straight line into space. Everything in the universe that has mass pulls on everything else. You actually pull on Earth just as Earth pulls on you, but because you are so much smaller, you cannot feel your own pull.
One common surprise is that gravity pulls on all objects the same way, regardless of weight. If you dropped a bowling ball and a basketball from the same height in a room with no air, they would hit the ground at the exact same time. Air resistance is what makes lighter things like feathers fall more slowly in everyday life, not a difference in gravity's pull. Understanding gravity is the first step to understanding why planets orbit, why tides rise and fall, and why you can always count on what goes up coming back down.