Suppose you are given a circle. Give a construction to find its centre.

1.   Let us consider any three points $P,Q$ and $R$ on the circumference of the circle. 

2.   Make chords $PQ$ and $QR$ by joining $P,Q$ and$R$.

3.   Draw $AB$ and $CD$, the perpendicular bisectors of $PQ$ and $QR$. They will intersect each other at $O$. 

4. $O$ will be the center of the circle.