Meaning: This sentence tells us that planets are composed of solid materials and gases.
Choice of Words: "Rocks" are solid, hard substances that make up the Earth's crust. "Gases" are substances in the form of air or vapor.
Alternative Expressions
Planets are like giant balls of rocks and gases.
Meaning: Planets are very large, round objects made up of solid materials like rocks and gases.
Choice of Words:Giant means very large. Balls are round objects. Rocks are hard, solid objects. Gases are substances that don't have a definite shape or volume, like air or helium.
Planets are made up of stuff that's hard like rocks and stuff that's like air.
Meaning: Planets are made up of a combination of solid materials like rocks and gases like air.
Choice of Words:Stuff is a general term for things or materials. Hard means difficult to break or damage. Rocks are hard, solid objects. Air is a gas that surrounds the Earth.