British Science Week is here and if you’re looking for some fun science demonstrations to amaze your class then here are good examples to show them. The children could carry these out too, perhaps as part of a circus of experiments around the classroom.

These work best if you use a bit of showmanship to dress them up as magic tricks – and then explain the science behind them.

Balance a penny on the end of a metal coat hanger

You’ll need: a metal coat hanger, a penny

Note: The end of the coat hanger will need to be filed flat. You’ll need to prepare this before the lesson.

1. Stretch the hanger from the middle so that it makes a square.
2. Place the bend of the hanger on your index finger and let it hang so that the hook is hanging down freely.
3. Balance a penny on the flattened end of the hook.
4. Slowly swing the hanger back and forth. The penny should stay in place.
5. Try and do a full loop-the-loop and see if the coin stays on.
6. Remove the coin at the end to prove to your audience that the coin was not glued on.

What’s going on? As the coat hanger is spun around, an accelerating force is applied to the penny which pushes it down onto the hook. According to Newton’s 1st Law, a force is applied towards the centre of the coat hanger, creating something called centripetal force.

The Levitating Ping Pong Ball

You’ll need: ping pong balls, a hair dryer

1. Turn the hair dryer on to the highest setting and point it straight upwards.
2. Gently hold the ping pong ball in the middle of the flow of air from the dryer and let go
3. The ball should levitate.
4. Explore how far can you tilt the hair dryer and keep the ping pong ball in the air?

What’s going on? This is called Bernoulli’s Principle. Air rushes over the curved surface, creating lower pressure and a pulling force away from the surface of the ball. The ball is curved, so the forces all around it are balanced. This keeps it from moving side to side. It’s the same thing that happens with an airplane wing: the rushing air causes lift which pulls the plane up.

Get a hard-boiled egg into a milk bottle

You’ll need: Glass milk bottle, hard-boiled/pickled egg, matches

There are three ways to do this trick.

1. Light a match and while it is burning drop it into a milk bottle. Place the hard-boiled egg onto the neck of the bottle. When the match goes out, the egg should be sucked into the bottle.
2. Pour a little hot water into the bottle and leave for 10 seconds. Place the egg over the neck of the bottle and allow the water to cool. To speed it up you could stand the bottle in a shallow bowl of cold water.
3. Put three matches into the top of the egg. Light the matches and, keeping the matches upright, quickly put the neck of an upside down milk bottle over the matches so that it touches the top of the egg. The egg should be sucked upwards into the bottle.

Tip: Remove the egg shell first! You can test this out with a small balloon filled with water instead of the egg. It’s easier to get it out again and repeat. Buy a jar of pickled eggs to save some time boiling and peeling eggs. You might also want to smear a little vaseline around the neck of the bottle to help the egg move.

What’s going on? The heat of the match (or hot water) makes the air inside the bottle expand. When it cools down again it contracts, this causes low pressure inside the bottle which sucks the egg down into it.

Hold a glass of water upside down without spilling any

You’ll need: a glass, a jug of water, a square of cardboard, a washing up bowl

Make sure the square of card is 2.5cm larger than the glass on all sides. Do this over the bowl, just in case!

1. Fill the glass with water so that it is completely full to the brim.
2. Place the card over the mouth of the glass. Make sure the glass is in the centre.
3. Hold the glass over the washing up bowl (just in case).
4. Place one hand on top of the card and quickly turn the glass upside down, holding the card in place.
5. Slowly remove your hand from the card. 

What’s going on? First, the pressure of the air is higher outside the glass than inside. This pushes the card upwards and holds it in place (if you get it just right). The surface tension of the water also helps to hold the card in place.

Instant Fire Extinguisher

You’ll need: a measuring jug, bicarbonate of soda, vinegar, a plastic spoon, candles and a candle lighter

1. Add a few spoons of bicarbonate of soda to a measuring jug.
2. Light some candles.
3. Fill the jug about a third full with vinegar. Allow the reaction to bubble for a while.
4. Carefully hold the jug above the candle flames. Slowly tilt the jug and pour some of the gas out. (Don’t pour the vinegar!)
5. The gas should extinguish the flames.

Safety: Wear safety goggles. Be careful with exposed flames. Do not inhale the carbon dioxide.

What’s going on? ? The reaction of bicarbonate and vinegar (acid) produces carbon dioxide gas. This is denser than air, so can be poured like a liquid. As it falls onto the candle, it pushes out oxygen, so the candle can no longer burn.

Make a sauce sachet rise and fall inside a bottle

You’ll need: a two-litre plastic bottle, a sachet of ketchup/mustard, water and salt (optional)

1. Fill the bottle right to the top with water. Put the ketchup sachet (still sealed up) inside the bottle.
2. If the ketchup sinks, add a few spoons of salt to the water.
3. Seal the bottle up tightly.
4. Squeeze the bottle. The ketchup should sink when you squeeze the bottle and float upwards when you release it. Try and see if you can get it to stop in the middle of the bottle.
5. Wow your audience with your ability to command the ketchup to rise and fall!

What’s going on? This effect is sometimes known as a cartesian diver. Basically there’s a small air bubble inside the ketchup sachet. When you squeeze the bottle hard, you put pressure on the ketchup which causes the bubble to get smaller. This makes the sachet denser than the water around it, so it sinks. When you stop squeezing, the bubble expands, making the sachet less dense again and so it floats back up.

Make a tea bag fly

You’ll need: a teabag (with a string and label), a ceramic plate, scissors, a lighter or match

1. Remove the staple, label and string from the teabag. Empty the bag out.
2. Unfold the teabag.
3. Open the teabag out into a cylinder.
4. Place the plate on a flat surface.
5. Stand the cylinder up on one end on a plate.
6. Use a lighter or match to set the top of the teabag cylinder on fire.
7. Watch as your teabag takes flight!

Safety: Wear safety goggles. Be careful with matches and naked flames. Don’t do this experiment near anything flammable. Watch how to do it here.

What’s going on? As the air inside the teabag gets hot, it expands and becomes less dense. The less dense air will rise up over the denser colder air. This is the same as a hot air balloon. The bag also creates a convection current of hot air which rises. Cold air rushes in, pushing the bag up. Planet Science has more here.

The Curious Case of the Disappearing water

You’ll need: several sheets of newspaper, scissors, a disposable nappy, paper cups, and a jug of water

You’ll need to do a bit of preparation before you perform this trick:

1. Spread a sheet of newspaper out.
2. Cut the nappy in half. Shake it out over the newspaper. You want the white powder from the nappy. Remove any fluffy cotton.
3. Pour the powder into one of the paper cups.

To perform the trick

1. Tell the audience you are going to make the water vanish.
2. Pour a small amount into the paper cup with the powder in it. The powder should absorb all the water.
3. Say the magic words. (You can choose what they are!)
4. Tip the cup – no water should pour out.

To bring the water back, pour some salt into the cup and stir. This will stop the powder working, and release the water.

Tip: Instead of cutting up nappies, you can also do this trick with Magic Snow, which is the same chemical. You can buy it on Amazon.

What’s going on? Nappies contain an absorbing gel called Sodium Polyacrylate. It’s able to absorb 200-300 times its own mass in water. When you add salt, this interferes with its ability to hold on to the water, so it releases it all.

Have you got any other favourite tricks? Put them in the comments below