ScienceGrade 3

Why Do Gases Fill Their Containers?

A gas has no fixed shape and no fixed size — it fills its whole container.

Learning Objective

Students will learn that a gas has no shape of its own — it spreads out to fill every part of whatever container it is in, because its tiny particles zoom around and bounce into every corner (TEKS 3.6B — classify matter as solids, liquids, and gases by shape and volume behavior).

What You'll Learn

Gas is made of tiny particles that zoom around fast and spread out in every
A solid keeps its shape; a liquid takes the container's shape but stays pooled
A balloon puffs up because the air inside pushes out in every direction as it
You can smell cookies across a room because their gas particles spread through

Words to Know

Gas: Matter that spreads out to fill its whole container — no shape of its own.; The air in a balloon is a gas.
Particles: The tiny invisible bits everything is made of.; Gas particles zoom around like a game of tag.
Container: Whatever holds something — a bottle, a bag, a balloon, a room.; The bottle is a container for the air.
Solid: Matter that keeps its own shape no matter where you put it.; The ice cube stays a cube in the box.
Liquid: Matter that takes the container's shape but pools at the bottom.; The water sloshes flat in the cup.

Check What You Learned

Quick Quiz

1 / 5

Which one fills its whole container — the ice cube, the water, or the air?

Fun Facts

A gas has no definite shape and no definite volume — it expands to fill every part of its container evenly
Source: TEKS 3.6B; standard physical-science curricula / NGSS matter definition
Gas is made of tiny particles in constant, fast, random motion — they move in every direction until they hit the walls, which is why the whole container fills
Source: NGSS 5-PS1-1 particle model of matter; kinetic theory for elementary grades
A solid keeps its own shape; a liquid takes the container's shape but keeps its own volume (pools at the bottom); a gas takes both the container's shape AND volume
Source: TEKS 3.6B state classification
A balloon inflates because the air particles inside spread out and push on the rubber in every direction
Source: standard classroom balloon/particle explanation
Odors travel across a room because scent molecules spread through the air as a gas — diffusion fills the available space
Source: standard diffusion example in elementary science texts
Air pumped into a bigger container doesn't settle at the bottom — it spreads thinner and fills the larger space completely
Source: gas expansion under the particle model; Boyle's-law-free elementary framing

Lesson Info

Grade 3

2026-10-23

Standards Alignment

TEKS 3.6B
describe and classify samples of matter as solids, liquids, and gases and demonstrate that solids have a definite shape and that liquids and gases take the shape of their container
5-PS1-1
develop a model to describe that matter is made of particles too small to be seen
2-PS1-1
describe and classify materials by observable properties