Take three hands-up answers, not open call-outs. Steer toward the idea that a single distance (across, or out to the edge) describes a circle's size. Do not name radius or diameter yet — the next step introduces the vocabulary on the board.

• First circle — name each part as it appears. Trace the circumference right around the edge with the cursor so pupils see the part they will label later. The diameter is the headline: it is always two radii laid end to end.
• Second circle — pause before the diameter shows; ask for a hand and hold out for double the radius on screen.
• Third circle — this is the reverse: given the width across, the radius is half. Ask what half the diameter is before pointing to the answer.

Keep diameter = twice the radius as the sentence pupils hear you say each time.

Talk this one through together — pupils take turns at the board and the class agrees or corrects out loud.

Drive the values in the order named. Start with radius 4. Then radius 7. Then set the radius so the diameter is 10 — this is the reverse move, so pupils choose radius 5. Finally drag to radius 2.5. Before each reveal, ask the class to predict the diameter. The 2.5 case is the wrinkle: pupils double a decimal, so hold out for five. Revoice a strong answer: so if I know the radius, I double it — and if I know the diameter, I halve it.

This is the practice round — pupils work each conversion in their copies, then the class confirms each answer aloud before moving on. Keep it brisk rather than over-explaining.

The circle-tool anchored the radius–diameter link in the last two steps; this is the written ×2 / ÷2 drill on paper. The diameter-9 line is the wrinkle: halving an odd number gives 4.5 cm — hold out for the decimal answer rather than 4 with a remainder. On the last item, watch that pupils halve 24 to get 12 cm rather than doubling by mistake.