Tool Impressions

The soda machine at your school has been broken into and the money has been taken. Hall monitors stops two students from another school in the hallway and fined a crowbar in one student’s backpack. How can your school prove that these students committed the crime? Try the following activity to investigate how detectives can match tools to the scene of a crime.

Materials

- hammer
- 16d nail (about 3 inches [7.5 cm] long)
- scrap piece of plywood
- pliers
- crowbar
- screwdriver
- timer
- adult helper

Procedure

NOTE: This activity requires adult help.

1. Have your adult helper use the hammer to pound the nail into the plywood so that ½ inch (1.25 cm) of the tip of the nail sticks out the back of the wood.
2. Leave the hammer, pliers, crowbar, and screwdriver near the plywood.
3. Tell your helper that he or she will have 3 minutes to remove the nail from the board, using several of the tools that are supplied.
4. Leave the room and time your helper for 3 minutes while he or she performs the task. Do not watch while your helper removes the nail.
5. When 3 minutes are up, try to match the marks made on the wood with the tools that were used. Which ones are easy to match? How could police use this information to investigate crimes?

Explanation

Almost any device used to perform work can be called a tool, and almost every tool will leave behind some mark of its use. There are three categories of tool marks: impression, such as those made by a hammer or a crowbar in a soft surface; cut marks, such as those made by wire cutters, saws, and the like on material; and scratch marks, such as those made by a knife blade scraping over a surface. Like other forms of trace about which tools were used at a crime scene. If the tools found on a suspect match the tool marks left at the scene of a crime, police have evidence to link the suspect to the crime.

One of the most frequently found tool impressions is the ‘jimmy’ mark found at the site of a break-in or forced entry. These marks occur when a criminal wedges a jimmy--a crowbar, screwdriver, or tire iron--between a door and its jamb, or between a window and its still, and exerts pressure until the door or window opens. The hard metal of the tool compresses the wood, leaving an impression of the tool in the wood.