For each volunteer, write a number on a piece of paper (starting at ‘1’), fold it up and place it in the bowl.
Give everybody two polystyrene cups. If the cups have raised edges, cut these off.
Instruct them to use the marker to write their name on one. The other should be left blank.
Ask the volunteers to bite (not all the way through) by placing the lip of each cup as far inside their mouth as they can and clamping down firmly with their teeth, leaving a light impression.
Collect the cups with their names on them and place them at one end of the table.
Before collecting the blank cups, ask the volunteers to pull a number from the hat, memorise it, and then place it into their unlabelled cup. Place all these unlabelled cups with the others.
Invite everybody to match the named cups with the numbers using just the teeth marks.
What’s happening?
By biting the cups, volunteers left behind an impression of their teeth. Just like our fingerprints, our dental patterns are all rather different.
Although they are white and made from similar chemicals, teeth are not bones. Rather, they are made from several different types of tissue that range from a soft dentine to an extremely hard material called enamel. Their shape and position are related to our diverse diet of meat, soft fruit and tough vegetables.
On average, humans have 20 primary or ‘milk’ teeth that fall out before adolescence and are replaced by 32 permanent teeth – half in the lower jaw (mandible) and half in the upper jaw (maxilla). They are divided into three groups – incisors for cutting, canines for piercing and grasping, and molars for grinding. Adults have a group of teeth called ‘pre molars’ that grow beneath the molar milk teeth, and can have even more molars at the very back called ‘wisdom teeth’.
The shape and growth pattern of teeth depends on a range of factors such as the size of the jaw, original positioning of the teeth, dental problems and dental work that has occurred over time. They also change rather slowly over time, meaning it’s possible to match a bite mark with a person, even if a long period of time has passed.
Applications
Forensic odontology is the term used to describe the field of study where bite marks are matched with people. Bite marks have been used as evidence since the late 19th century where a victim has been bitten or where an object – such as a piece of fruit – has tooth impressions. One of the first published cases involving forensic odontology occurred in Texas in the 1950s, when a lump of cheese was used to link a suspect to a burglary.
Some argue that although bite marks differ between most people, there isn’t much evidence showing our mouths are all completely unique, leading to the possibility of false convictions. It might also be possible to misread impressions where the material has stretched or changed shape.
Because of the tough coating of enamel on our teeth, they often survive a fire where bones don’t. If the teeth can be matched to a dental record, it’s possible to identify a victim this way.
That reminds me – it’s time I booked my next dental appointment.
