In the 1760s, the English lawyer Sir William Blackstone (1723-1780) distinguished between “infants” (up to the age of seven) and “adults” when judging whether someone should be held responsible in law for their actions. He argued that very young children can’t form an “intent”, so they wouldn’t know or fully understand the consequences of their actions. Suppose you set fire to a wooden shack because you want to see flames, or keep warm. As an adult you would know to check there was no one inside. As an “infant”, it would not occur to you that burning some wood might kill someone. The child shouldn’t be held responsible for the consequences as they didn’t intend them and couldn’t be expected to anticipate them.

But how to know whether a child was under seven years old? Today, most of us have birth certificates and can say exactly when we were born. But in the UK, there was no official procedure for registering births until 1837, and even then it wasn’t compulsory. With no more reliable guide, they looked at a child’s teeth. Conveniently, the first adult molars (back teeth) emerge around the age of seven. It became an indicator: if you had any adult molars, you could be held responsible for your actions. So, if you’re young and want to burn down a wooden shack, check your teeth first. (Better still, check inside the shack.) Any molars? Put down the matches.

Find out more about criminal responsibility.