Learning Aims:
    • Students apply the process, which takes place in nature (water filtration through layers of rocks) to the given problem.
    • They explain the concept of filtration.
      • Worksheet, PET bottle, sand, hay, crumpled paper, a glass of dirty water (soil with vegetation poured with water), a large empty container, matches, soap, etc.
      Suggestions for use:
      • Motivation: Imagine that you are in South Asia. You are very thirsty and the only water available is muddy water from puddles. How to purify this water to make it drinkable? You have only these things: PET bottle, sand, hay, crumpled paper, a glass of dirty water, a large empty container (ideally a 3-litre jar), matches and soap (items can be differently replaced or others can be added).
      • Task: Students work in groups and their task is to build a water purification device using the above items.
      • Solution: This is rough dirt filtering through the sand filter (emphasize that this process also occurs in the nature). The water that soaks into the ground is naturally cleaned when passing through the rock layers.
      • Question: Is the water cleaned in this way drinkable? Healthy?
      • Answer: No!!! A variant how to partially "disinfect" water without chemistry is to leave it in a transparent container in the intense sun. UV light will destroy germs or to boil water over the fire.


      • We will ask the students to propose the way how to find out the (un)safety of filtered water.
      Possible questions:
        • Students formulate their own questions. A large amount of questions arise through this activity.