John Jones knew he wanted to be a scientist from the age of four. Actually, he knew in his bones long before that, but he didn't hear the word "scientist" until he was four and even then he hardly knew what it meant.
When John was given his first proper toy, not counting all the cuddly animals which lay in his cot when he was a baby, he took it to bits. It was, as it happens, a tractor, since John lived in the country and tractors were very familiar to him.