What a way to start the day! Opening your desk, you find your treasured chocolate bar has gone. In its place is a ransom note - “if you want to see your chocolate bar unbitten againI” and some clues. The thief has left a fingerprint, and a footprint - fortunately in a tray of modelling clay.

There are some suspects. Chocoholic Chris is one; so is Sweet-tooth Sam. Then there is Munching Mick and - well, you think them up! And there is some evidence against each of them: a favourite pen, which might have been used to write the ransom note, a trainer, and some fibres from a jumper. But can your class pin the crime on one of the suspects? They will be forensic scientists for the day - and incidentally learn something about materials.

Suspect shoe

Press the culprit’s shoe into a tray of soft modelling clay or damp sand. Provide several trainers with different patterns. They can probably be matched on sight. (Lighting from the side throws the sole pattern into relief.) A permanent record can be made by pouring a mixture of plaster of Paris into the impression. Leave it for 24 hours to set. Notice that mixing plaster and water produces heat because there is a chemical reaction. Match the sole to shoes from the lost property box. Conviction looms!

Foul fibre

Use a binocular or monocular microscope to match fibres by sight. Use a fibre from the crime scene, and a variety of others. Colours, thickness, kinks and other peculiarities will help to close the case.

Which pen?

Which pen wrote the ransom note? Black felt-tipped pens contain ink made up from an amazing range of dyes - including yellows and deep purples. You need water-based pens (reject any that smell of other solvents). Write the ransom note with the culprit’s pen on soft absorbent paper such as kitchen towel, and then mark this pen and several others with the suspects’ names.

Cut the ransom note into pieces for pupils to investigate. Adding a few drops of water will cause the ink to spread through the paper and the coloured dyes will separate out. Repeat the process using the other pens and you get different blots of colour. Match the patterns of colour and you are on your way to a conviction.

Which thumb?

Fingerprints take well on very smooth surfaces such as glass. Take a tumbler, wash it thoroughly and then press the culprit’s thumb on to the surface. From now on, handle it - like Sherlock Holmes - with a handkerchief. Use an ink pad and some willing colleagues to provide some fingerprint records to match. Label each with the name of the suspect, making sure that you have included the culprit. Ask the children to use hand lenses to match the loops and whorls. They will need to shake a little talcum powder on the tumbler print and blow it gently. It will stick to the pattern. Another piece of evidence!

This chocolate is alarmed!

The crime is solved. Munching Mick will be going down for a long stretch. But how to stop the choc bar disappearing again? The answer is to “alarm” it - a challenge from the QCA science scheme of work unit 56H and from design technology unit 4D. You can alarm the classroom door, fix a pressure pad that the burglar might step on, or make sure that lifting the choc bar rings a bell.

door Alarm: Tin foil taped to the door jamb makes an effective alarm switch. Opening the door sets off the light or bell. You might invite ideas for a door or window sensor; an alarm operated by breaking a magnetic field - use a tiny, magnetically operated reed switch; or an alarm triggered by moving something (make a tilt switch by wrapping a ball made from modelling clay in kitchen foil).

PRESSURE PAD: The pressure pad is a familiar challenge. Pressure switches work when two conductive surfaces - two sheets of cooking foil, for example - are brought together to complete a circuit. In order that they shouldn’t touch accidentally, but only when someone stands on the switch, they are separated by a squashable insulating material such as sponge. Holes in the sponge allow the two conductors to come into contact. Hobby shops sell strips or rolls of grey foam as the underlay to model railway track. This can be used to make a variety of patterns of holes, so that it becomes possible to compare the effectiveness of alarms, not just to find that some work and some don’t. You can also use foam pipe insulation - the tubular stuff you put round cotton reels to make realistic wheels. Slice it very thinly with a junior hacksaw, holding it in a vice for safety. Lay the slices on their side, rather like decorating a pizza with tomato.

alarm the bar: Many chocolate bars are sold in a tube of wrapped material with a loose “seam”. If the bar is placed in a spring-loaded alarm with this seam in the jaws of the switch, the alarm will be operated if the bar is taken away - removing the bar completes the circuit. You can also use commercial “push to break” micro-switches. These are the kind that you find in fridge and washing machine doors. Lifting the choc bar - like opening the fridge door - completes the circuit and the alarm goes off. If the choc bar isn’t heavy enough, tape it to a wooden brick.

WHICH ALARM IS BEST? Compare the different systems. While a bell or buzzer is fine, you may find an alarm light is just as effective and more restful.

Safety note: be aware of diabetes or other dietary needs if you let children eat the evidence.

You can buy ready-made pressure pads. An interesting exercise is to examine them to see what materials they are made from and how they are constructed (TS-3 Pressure Pad, price pound;6.92 each from Technology Teaching Systems, 0800 318686).The Carolina Biological Supply Company provides a forensic science kit (Crime Scene 1, pound;78 + VAT, Catalogue number 69-9819, supplied by Instruments Direct, telephone 020 8560 5678 or e-mail sales@instrumentsdirect.co.uk. Their blood sample test uses fake blood - real blood is NOT recommended for safety and health reasons.)