Video narration
The house is still unfinished, and your compass is set to five centimetres. With that opening, let's find a point equally far from B and C. First, place the compass point at B and draw an arc with a radius of five centimetres. Keep the arc visible. Without changing the compass opening, place the compass point at C and draw another arc. Now the two arcs meet above B and C. That upper meeting point is A. Here's why: A lies on the arc centred at B, so AB is five centimetres. A also lies on the arc centred at C, so AC is five centimetres too. The unchanged compass opening gives both arcs the same five-centimetre radius, and A sits on each one. So AB equals five centimetres, and AC equals five centimetres. We have found the point that is equally distant from B and C!