Bottle Thermometer

What you need

• A bottle with a screw top
• A transparent straw
• Colouring (water-based paint or food colouring)
• Some modelling clay  
• A felt-tip pen (waterproof)
• Scissors

And this is how it works

Fill half of the empty bottle with water and add some colouring. Tighten the bottle top and shake the bottle until the colouring has completely dissolved. Then fill the rest of the bottle with water. The bottle should be as full as possible. You will now probably need the help of an adult: use the point of the scissors to carefully (!) make a hole in the bottle top. The hole must be big enough for the straw to fit in.

Now push the straw through the bottle top until it hangs a few centimetres in the water. Seal the hole in the bottle top around the straw with modelling clay until you are sure that no air can get into the bottle.

Now unscrew the bottle top, because some air has to be let into the bottle. Carefully suck on the straw until the water inside it rises around five centimetres above the bottle top. Tighten the bottle top again and keep the water in the straw, for example by using your tongue to keep the straw sealed.

You now need the felt-tip pen: mark the water level on the straw, this is your room temperature. You can now place the bottle on a sunny window sill or in your fridge. The water will go up in the sun and go down in the fridge. If you mark the water level each time you can then roughly take a reading of how warm or cold it is. Have fun experimenting!