My 5 year old asked me this question and I was unable to explain why!
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Here’s a fun activity to do with the kid: build a tower as tall as you can before it falls over. Use any materials you like, but I like marshmallows and dry spaghetti. Experiment with different designs; help him notice that the wider the base, the taller the tower can be. Explain also that things fall because gravity is pulling them down.
Now, talk about mountains. Like the tower, if a mountain gets too tall, gravity will pull it down. But gravity doesn’t really pull “down”, it pulls toward the center of the mass (you might need to explain that “mass” is “stuff” (which is actually “matter” and mass is how much stuff there is, but this nuance can be explained in a couple more years)). This is why Australians don’t fall off.
The earth is round because mountains can only get so tall. A cube would have eight mountains that are too tall, and they would get squished down by gravity until we had a ball again. Same with any other shape with corners. Even a flat disk; the edge would just be a really tall and long mountain range that would get pulled towards the center until you had a ball.
Here’s another activity: try to make a square drop of water. Use a piece of wax paper or other hydrophobic surface, and place a drop of water on it. Use a toothpick to try to change the shape of the drop. As soon as you take away the toothpick, the drop will go right back to being round. All the little tiny (so tiny we can’t see them) bits of water (called molecules) are pulling on each other in all directions. This means that any corners get pulled in until you have a ball. Just like all the rocks that make up Earth.
You can also try blowing square bubbles. Same principle.
Have him hold a few things that are heavier, and then explain that all the rocks in the Earth attract all other objects and pull them down, and that’s what gravity is. It’s a property that all materials, not just rocks, but also water and air and everything, attract all other materials in the vicinity.
So the Earth is shaped like a ball because that’s the shape that happens when every material tries to squeeze itself into the smallest space. Like if you take a bunch of snow and try to squeeze it into the smallest space, you get a snow ball; a ball is the shape that holds the most material in the smallest space.
If he has questions about how matter can attract matter, just explain that this is how things are; scientists have studied gravity and he’ll learn about it in school. But as another example, get some magnets and explain that magnets attract other magnets just like gravity, except that it’s not gravity it’s magnets, but he can play with the magnets and feel the force between them, and then when he holds the heavy objects again, he’ll get that gravity pulling down on the heavy object is the same as magnets pulling each other close.
Gravity. Gravity pulls us down into the earth, but it also pulls the earth down into the earth. This means the earth is quite literally being crushed from all directions. Imagine a ball of play-doh that you cup in your hand and push from all sides. The result is going to be roughly ball-ish.
A planet is round because of gravity. A planet’s gravity pulls equally from all sides. Gravity pulls from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere, which is a three-dimensional circle…
Copy and pasted this from Google..a very good source of information
Essentially because of gravity. Gravity pulls everything on and in the Earth towards the center of the Earth. The shape which has as much stuff as possible as close to the center as possible is a sphere, which is why the Earth (and pretty much all other large celestial bodies) is round.
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