Every part of a circle starts at one point.

The centre, and the rope called the radius.

The centre is the fixed point right in the middle of a circle. It isn't part of the curve itself — it's the anchor everything else is measured from. Mark it with a dot, and you've planted the flag the whole shape grows around.

The radius is a straight line from the centre out to any point on the edge — and we also use the word for that line's length. Here's the magical bit: the radius is the same in every direction. North, south, east, a weird slanted angle — measure from the centre to the edge and you'll get the exact same distance every time. That unwavering sameness is precisely what makes a circle round instead of lumpy. (If you ever draw two or more of them, the plural is "radii," said ray-dee-eye.)

This is exactly how a compass draws a circle. The sharp point pins the centre to the paper, the pencil sits a fixed distance away — that gap is the radius — and as the arm swings around, the pencil can only ever trace points that stay that one distance from the point. Widen the compass and you've simply chosen a bigger radius. So a circle isn't really drawn by following a curve; it's drawn by obeying a rule about distance.

Hold those two words tight — centre and radius — because in the next section you'll click on a real diagram and watch every other part of the circle introduce itself, each one defined by how it relates to these two.

Click the circle. Light up every part.

This is the heart of the page, so play with it before you read on. Tap a name below — or click the glowing dot for the centre, or the rim for the circumference — and that part lights up on the circle while its definition slides into the card underneath. Work your way through all of them. Notice how every single one is described using the centre or the edge: that's the family resemblance in action.

The diameter is always twice the radius.

The diameter is a straight line that crosses the circle from one side to the other, passing right through the centre. Because it goes centre-to-edge in both directions, it's built from two radii laid end to end — so it is exactly twice the radius, every time, in every circle. No exceptions, no rounding. We write it as d = 2 × r.

This relationship is a genuine shortcut. Measure a pizza straight across the middle and you've found its diameter; halve it and you instantly know the radius, with no second measurement. Or go the other way — know the radius of a bike wheel and double it for the diameter. The two numbers travel together, locked at a ratio of exactly two to one.

Drag the slider to set the radius and watch the diameter readout. It refuses to be anything other than double. There's a bonus readout too: the circumference — the distance all the way around. It comes out roughly π times the diameter, and we'll use the friendly estimate π ≈ 3.14, so circumference ≈ 3.14 × d. (More on that mysterious π in a moment.)

A chord is not a diameter (usually).

A chord is a straight line joining any two points on the circle. That's the whole definition — it does not have to go anywhere near the centre. Stretch a guitar string across a circular drum at any old angle and you've made a chord.

So where does the diameter fit? The diameter is simply the one special chord that happens to pass through the centre — and because it does, it's the longest chord you can possibly draw. Every diameter is a chord; almost no chord is a diameter. Picture all the chords you could draw as a big family; the diameter is the one tall sibling that always walks straight through the middle.

The arc is the crust; the sector is the slice.

Now the two tastiest parts. An arc is a piece of the circumference — a curved stretch of the edge between two points. On a pizza, the arc is the curved crust along the outside of one slice. It's not a straight line like a chord; it follows the bend of the circle.

A sector is the whole pie-slice region trapped between two radii and the arc that joins their ends. Cut a pizza from the centre out to the rim twice, and the wedge you lift away is a sector — two straight radius edges and one curved arc edge. A slice of pie, a wedge of orange, a piece of pinwheel: all sectors. So the arc is the curved edge, and the sector is the filled-in shape behind it.

Hiding in every circle: the number π.

Here's a question that sounds too simple to be interesting: how many diameters would it take to wrap all the way around a circle? Roll any circle along the ground for one full turn — a coin, a dinner plate, a bike wheel — and measure how far it travels. That distance is the circumference. Now compare it to the diameter, and you get a little shock.

It's always about 3.14 diameters. Not 3, not 4 — three and a bit, the same "bit" for a button and for a planet. That ratio of circumference to diameter is so important it has its own name and its own symbol: π (the Greek letter "pi," said like "pie"). In short:

circumference ÷ diameter = π ≈ 3.14

π is actually 3.14159… and never stops or repeats — but 3.14 is plenty close for us.

That's why the diameter slider earlier multiplied by 3.14 to estimate the distance around. You don't need to master π today — just meet it and notice it lives in every circle. When you reach the circumference and area pages, π will be the key that unlocks both. For now: every circle, no matter its size, is exactly π times as far around as it is across. One number, every circle.

Name that part.

Time to prove the names have stuck. One part of the circle lights up; you tap its name. Get it right and your streak climbs. Get it wrong and the right answer reveals itself, so you learn either way. Press Next part for a fresh one — there are eight in the mix.

Four quick questions.

The game tested whether you can recognise a part by sight. These four check that you've really got the relationships — the doubling, the chord trap, the slice, and π. Pick an answer and you'll see why it's right or wrong straight away.