An astronomer with his very own cometary nucleus and some very excited Aberdeen schoolchildren. Image: Dr Francisco DiegoOne of the neatest demonstrations in astronomy is to make a comet in the classroom. Using relatively simple materials, you can build a model of these exotic objects in a few minutes. Students are likely to remember the inevitable mess, bad smells and noise that go with it.

The basic ingredients:

• 2 cups of water (about 2 cups for a smallish nucleus)
• 2 cups of dry ice (frozen CO₂)
• 2 tablespoons of soil or sand
• a dash of ammonia (use the crystals from a bottle of smelling salts or alternatively cleaning fluid)
• a glug of 'organic material' – dark soy sauce or Worcestershire sauce are ideal

You’ll also need the following equipment:

• an ice chest (or at least a polystyrene container to insulate the dry ice until it’s used – they can be obtained from a greengrocer)
• a mixing bowl or washing-up bowl
• a rubble sack or thick garden waste sack
• work gloves (the thicker the better – use the sort that are coated in plastic)
• a rubber / wooden mallet or hammer
• a large wooden spoon or spatula
• a few floor cloths
• a set of kitchen scales
• safety glasses

Dry ice can be bought commercially from ice suppliers, although normally they have a minimum order. It's worth ordering a little too much to give you a chance to practice this before the final demonstration.

Alternatively, you can make your own by venting a carbon dioxide cylinder. Instructions are available on the Practical Physics website.

Warning: solid carbon dioxide should never be stored in a sealed container as there is a risk of explosion

To make the comet:

• Place a rubble sack in your bowl and open it out
• Pour the water into the sack
• Add the soil or sand and stir it into the water
• Add the ammonia – from this point your mixture has a fairly pungent aroma
• Add the 'organic material' – a good glug of sauce should do the trick
• Put on the work gloves
• Place the dry ice on the floor cloths – this protects the surface underneath and makes it easier to add to the water
• Pound it carefully with the mallet or hammer until it resembles a fine powder
• Carefully pour the powder into the sack, stirring the mixture vigorously – at this point the blend will hiss and pop as the water freezes and the dry ice sublimes
• Use the work gloves to squeeze the comet through the sack and shape it into a round ball – you’ll need to press hard to make the ice stick together
• Unwrap the sack and you should have a large dirty ball, with small jets of CO₂ fizzing out through the holes in the water ice crust
• It's worth inviting students to take a closer look and perhaps prod the comet with sticks – although it's fairly safe to handle any areas of exposed dry ice could burn the skin
• At this point I tend to place the cometary nucleus on the scales – they typically weigh in at about 0.5 kg. Travelling towards the Earth at a speed of 70 kms-1, it would have the same kinetic energy as a Boeing 747 (jumbo jet) at take-off.
• Finally you can break the nucleus open – either using the gloves or (gently) with the hammer. Most of the CO₂ sublimes fairly quickly, leaving a nucleus full of holes like a Swiss cheese. This is probably the way that many comets end up as their volatile material is exhausted by successive close passes to the Sun.
• If you wait long enough the dominant remaining material becomes the sand or soil you started with – some extinct comets have this appearance and become rubble piles weakly held together by gravity