Skip to content
Spark & Whetstonethe grown-up commons

← Things That Run

Things That Run

Three short lessons on systems your child explores in science: the water cycle, a food web, and a logic gate. Each one runs by itself once it is set going.

Spark & Whetstone · sparkwhetstone.org/learn/things-that-run · CC BY-NC-SA 4.0. Answers are at the end, so you can try each question first.

Lesson 1. The water cycle

The Sun warms water in seas and lakes, and some of it evaporates into the air as vapour. Higher up, the air is cooler, and the vapour condenses into tiny droplets that make clouds.

When the droplets grow heavy they fall as rain or snow, run into rivers and soak into the ground, and flow back to the sea. Then it starts again. Nothing is used up; the same water goes round.

Question. What turns vapour in the air into cloud droplets?

  1. The air cooling as it rises.
  2. Wind pushing it together.
  3. Salt in the sea.

Say it back. In your own words: what keeps the water cycle going?

________________________________________________

________________________________________________

Lesson 2. A food web

A food web shows who eats whom. Plants make food from sunlight; rabbits eat plants; foxes eat rabbits. Each arrow is a link.

Change one link and others move. If the foxes vanish, the rabbits may grow in number and eat more of the plants. Scientists studying the return of wolves to Yellowstone in 1995 have argued for years about how far effects like this spread, which is a good reminder that a web is easier to draw than to predict.

Question. In a simple web of plants, rabbits and foxes, the foxes disappear. What is most likely to happen first?

  1. The rabbits increase.
  2. The plants increase.
  3. Nothing changes.

Say it back. Say it back: why does changing one link change others?

________________________________________________

________________________________________________

Lesson 3. A logic gate

A logic gate is a tiny decision made with electricity. An AND gate turns its output on only if both its inputs are on. An OR gate turns its output on if either input is on.

You use them every day. A doorbell with buttons at the front and back door works like OR: either button rings it. A machine that needs both hands on two buttons before it starts, so a hand cannot be in the way, works like AND.

Question. A light should come on if either of two switches is on. Which gate is that?

  1. An AND gate
  2. An OR gate
  3. Neither kind of gate

Say it back. In your own words: what is the difference between an AND gate and an OR gate?

________________________________________________

________________________________________________

At the table: A water cycle in a bag

Players: a grown-up and a child, 5 or older. You'll need: a clear zip-lock bag, a little water, a marker and a sunny window.

  1. Draw a sun and a cloud on the bag. Pour in a few spoonfuls of water and seal it.
  2. Tape the bag to a sunny window.
  3. Check it over a few hours: droplets form near the top and run back down.
  4. Ask the child to point to evaporation, condensation and rain on the bag.

Answers

  1. The air cooling as it rises. — As the air rises it cools, and cool air cannot hold as much vapour, so it condenses into droplets.
  2. The rabbits increase. — With fewer foxes eating them, rabbits are likely to increase, and then eat more plants.
  3. An OR gate — If either input is enough, it is an OR gate.