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Summer Brain Games

Experiment: Gravity Sewer

What happens when you flush the toilet? Where does the water go? Build a gravity sewer to move water from one room of your house to another.

Instrucciones en Español


Paper cups
Straws (try crazy or bendy straws for an added challenge!)
Sharpened pencil, about the same diameter as your straws
Modeling clay
Food coloring
Bowl or basin


  1. Decide where you want to begin your gravity sewer challenge. A bathroom or kitchen works well, or even outside where you can use a hose. Once you know where you want to start, decide where you plan to end. The farther you travel, the bigger the challenge.

  2. Use a sharpened pencil to poke a hole in the side of the cup near the bottom. Push a straw through the hole; it should be snug. You can try to waterproof the hole by using modeling clay to plug up any leaks. These straws are the pipes of your sewer. Make sure the angle of the straw slopes downward so the water flows out.
  3. Add another straw to make the pipes longer. Push the end of a new straw into the end of the first. You can make this easier by using scissors to make a small lengthwise cut up the end of the straw. Overlap the two edges of your cut to make the diameter of the straw smaller so it can fit snugly into the first. Cover the connection with tape to waterproof it.

  4. Place the cup on a high spot, like a counter or table, so the angle of the straws continue to slope downward. Place a bowl or basin under the last straw, perhaps on the floor, to collect the water that runs out.

  5. Test your connections by pouring some water into the cup. If you discover leaks, waterproof them with more tape or clay.

  6. Continue adding straws to make the water travel farther. You may need to support the straws as they extend longer. Add support by taping the straw to objects along the way, like chairs or walls.

  7. Once your sewer is built, pour water in the cup and watch it travel. Add some food coloring to the water so it’s easier to see! How many straws can you use? What is the length of your sewer?

What’s Happening?  

When you flush a toilet, underground pipes or tunnels move wastewater to treatment plants where it can be cleaned and returned to the environment. Gravity sewers move wastewater using the force of gravity. Gravity is a natural occurring force that pulls objects towards each other – anything with mass has a gravitational pull. 

The Earth’s gravitational pull keeps our feet on the ground and makes objects fall back down when we throw them. Gravity sewers use the Earth’s gravity to pull wastewater lower and lower. This system reliably moves water with low energy costs and little maintenance!



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