A formula is a recipe card.

Atoms are the building bricks. Molecules are what you build.

An atom is the smallest piece of an element — a single unit of hydrogen, or oxygen, or carbon. On its own, one atom usually doesn't do much. But atoms love to join up. When two or more atoms clip together and stay together, you get a molecule — a brand-new thing with its own personality.

Two hydrogen atoms and one oxygen atom, snapped together, become water. Not "wet hydrogen with a side of oxygen" — genuinely water, which you can drink. The joining changes everything. That's the magic worth remembering: the same atoms, arranged into a molecule, make something new.

Chemists give each kind of atom a one- or two-letter symbol so they don't have to write the whole name every time:

• H — hydrogen. The lightest, most common atom in the universe.
• O — oxygen. The one you breathe; it also lives inside water.
• C — carbon. The backbone of every living thing, from you to a tree.
• N — nitrogen. Most of the air around you is made of it.

Those four are all you need for today. Keep their colours in mind — you'll be clicking them together in a minute.

The little number is a counter.

Here's the one rule that unlocks every formula. Look at water:

H₂O

That small, low number after the H is called a subscript. A subscript tells you how many of the atom just before it. So the little 2 belongs to H, and it says: two hydrogen atoms.

What about the O? It has no number after it. And here's the second half of the rule: no number means one. You never write a little "1" — a lonely symbol already means exactly one of that atom. So O is one oxygen.

Read the whole recipe out loud: "H-two-O — two hydrogens, one oxygen." That's it. That's water, decoded. Every formula you'll ever meet works exactly this way: find each symbol, read the little number beside it (or read "one" if there's none), and you have the full ingredient list.

Snap atoms together. Watch the formula count itself.

Pick a target, then click atoms to add them to the workspace. They snap into place and the formula updates live. Click any atom in the workspace to pop it back off.

Same rule, three real molecules.

Now that you can decode a subscript, let's read a few more recipe cards. Tap a formula and it splits into its ingredient list — exactly the way you'd read it aloud.

Notice the pattern in each one:

• CO₂ — carbon dioxide. One carbon (no number = one), two oxygen. It's the gas you breathe out, and the gas plants breathe in.
• CH₄ — methane. One carbon, four hydrogen. The main ingredient in natural gas — the flame on a stove.
• O₂ — oxygen gas. Two oxygen atoms, nothing else. Even a molecule made of just one kind of atom still gets a formula. The little 2 reminds you oxygen in the air travels in pairs, not as loners.

Four molecules, one rule. Symbol tells you which atom; the little number tells you how many. Miss the number and it's "one." You've now read every recipe on the page.

Where does the little number go?

1. The subscript sits after the atom it counts — never before. In H₂O, the 2 comes right after the H, so it counts hydrogen. If you saw CO₂, the 2 sits after the O, so it counts oxygen — one carbon, two oxygen. The number always looks backward, to the symbol on its left.

2. A subscript only counts the one symbol right before it. It doesn't reach across the whole formula. In CH₄ the 4 counts only the H — you get one carbon and four hydrogen, not four of everything.

One more note for the curious: some formulas describe crystals instead of tidy little molecules. Table salt is written NaCl — one sodium (Na) to one chlorine (Cl). It isn't a lone two-atom molecule floating about; it's the repeating ratio inside a giant salt crystal. The recipe idea still holds perfectly: it's telling you which atoms, and in what proportion. For everything on this page, though, "which atoms and how many" is all you need.

Can you read the recipe now?

Four quick questions. Pick an answer — you'll find out right away.