What you’ll need

  • Two empty 0.5-liter plastic bottles
  • A measuring jug with water
  • Knife or scissors
  • A cutting board
  • An awl
  • Watercolor paints and paint brush
  • A waterproof felt-tipped pen
  • A stopwatch

How to do it

1. Ask an adult to cut off the top part of the bottle and keep the lower, cylindrical part. This will be the “receiving bottle.”

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2. Now ask your adult helper to cut off the lower part of the second bottle and keep the top part with the cap. This will be the “funnel bottle.”

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3. Unscrew the cap of the funnel bottle and make a small hole in the center using an awl. Ask an adult to help you. Screw the cap back onto the funnel bottle.

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4. You now need to calibrate your water clock, i.e., mark off the water level at different times. To do so, place the receiving bottle upright on a level surface and position the funnel bottle cap-down in the opening of the receiving bottle.

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5. In a measuring jug, mix approx. 300 ml water with watercolor.

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6. Pour the water into the funnel bottle and quickly mark off the water level before the water starts pouring through the hole.

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7. The water should start dripping into the receiving bottle. As soon as the water level reaches the cylindrical part* of the receiving bottle, make a mark with the felt-tipped pen. This will be the “zero point.” At the same time, start making a note of each 30-second interval. (* the cylindrical part that starts a few centimeters above the base)

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8. Now mark off the receiving bottle at 30-second intervals.

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9. Next, pour the water in the receiving bottle back into the measuring jug.

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10. Now start over and fill the funnel bottle up to where you marked it earlier. This time you won’t need the stopwatch; just let the water drip. What happens? The water level will reach the zero mark and then the other markings, each of which represents an interval of 30 seconds. Why do this? Because it lets you measure time! For example, you’ll be able to time a 3-minute egg or know exactly when you’ve brushed your teeth for a full two minutes.

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11. What you can also try out: fill the funnel bottle until it is only half-full. Begin timing with the stopwatch and check the water level every 30 seconds. What do you notice?

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Well spotted!

  • If you look carefully, you’ll notice that the gaps between the markings you made are not exactly equal. The emptier the funnel bottle gets, the closer the gaps. This is because the water flows through the hole faster at the beginning than at the end.
  • If the funnel bottle is only half-full, you’ll notice that 30 seconds of flow time do not match up with the markings you made the first time around when the funnel bottle was full: after 30 seconds the first marking still hasn’t been reached.

What's the secret?

It’s not just the size of the hole that determines how quickly the water flows: the shape of the funnel (the top of a bottle) also matters. The water would flow differently if the funnel were cylindrical like a tube or round like a wide bowl. The way the water flows depends mainly on the pressure of the water above the hole. The more water in the container, the higher the pressure above the hole. Logically, the water flows more quickly at the start, when the container is full.