Pierce through the middle of the carton carefully using the point of a small screwdriver or scissors.

Carefully cut around the middle of the carton and trim off any ragged pieces to leave two separate parts.

Place the plastic top from the carton on a thick piece of card or old cork tile and use a screwdriver or bradawl to pierce lots of holes in it. about 10 holes is fine.

Use scissors to cut a tiny piece from the edge of a dry new teabag and let the tea pour out into a cup or straight into the bin.

Place the paper teabag over the mouth of the plastic carton and carefully screw the cap back on so that the paper is trapped - no need to tighten it!

The paper teabag is the important part of the filter. It lets water through but will trap any solid particles. Teabags are good at this! Their job is to let the hot water get into the tea leaves without letting the tea leaves escape into the cup!

Now turn the top half of the carton which contains the filter upside down and gently fit it into the bottom half so that it rests in it - do not push it down all the way or this will stop the filter from working properly.

You can test the filter by making a mixture of sand and water or soil and water in a plastic cup and then pouring a little at a time into the home made filter.

Be patient while the mixture slowly filters through.

The soil or sand gets trapped on the filter paper (teabag) and the water passes through.

After a few minutes the water has all passed through and the top half of the filter can be lifted out and the screwcap removed.

The clear water is found in the bottom half of the milk carton.

If the water is a little yellow in colour then it is likely that the soil or sand contains something which has dissolved in the water. This can be removed but the method is complicated.

This picture shows the screwcap and filter paper removed.

You can see the sand which has been trapped on the filter paper.

Here is the clear water which has passed through the filter.