At first we looked at these triangles on a big scale. The school pitch, like every football pitch, has a very convenient circle, with a straight line right through its middle. If that line is the long side of a triangle, what angle would the opposite corner have?
Next we used a pair of compasses and a ruler to recreate the situation:
We've also started using GeoGebra in Year 5. It's great for showing how the angle is always 90°:
I didn't take it on to this with Year 5, but there is a fairly simple proof that this will be always the case...