Monday

Rocket drags a crate of plastic bottles into the workshop. "I have an idea!" he says. "Empty bottles float, so I will build a raft."

Raven picks up one bottle and turns it in the light. "You keep saying empty," she says. "What do you notice inside?"

Rocket squints. "Nothing. Just air."

Nova hovers over the crate, her cyan lights blinking. "Interesting," she says. "Is air something, or nothing? Try squeezing a bottle with the cap on."

Rocket squeezes. The bottle pushes back against his hands. "Huh," he says. "Something in there is fighting me."

Scientists use the word matter for anything that takes up space and has weight.

A rock is matter. Water is matter. The air in Rocket's bottle is matter too, even though you cannot see it.

Air is a mix of gases, such as oxygen, nitrogen and carbon dioxide. You breathe these gases every day.

All matter is made of extremely tiny particles called atoms and molecules.

An atom is the smallest building block of matter. A molecule is two or more atoms connected together.

How small are they? Millions of them would fit in the space of the period at the end of this sentence.

So we cannot see particles. Instead, we look for clues that they are there.

Scientists cannot see atoms, so they look for evidence instead.

A capped bottle that pushes back is one clue. Something inside takes up space.

Feeling your breath on your hand is another clue. Air can push even though it is invisible.

Great thinking today. You found out that "empty" is not really empty. Tomorrow we will see how particles act in solids, liquids and gases.