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Make a cube out of straws and have a go at this practical challenge.


Reasoning about the number of matches needed to build squares that share their sides.

Little Boxes

How many different cuboids can you make when you use four CDs or DVDs? How about using five, then six?

World of Tan 13 - A Storm in a Tea Cup

Age 7 to 11
Challenge Level

This activity follows on from World of Tan 12 - All in a Fluff.

The peaceful tea break at Granma T's is being transformed into a scene that could be from a pantomime. Wai Ping and Wu Ming, two of the workers, are arguing with each other...

Wai Ping: It sticks out.

Wu Ming: No, it goes in.

Wai Ping: Out!

Wu Ming: In! I'll prove it by asking the others.

Mah Ling is the first to come in for her break, and she is immediately stopped by the workers.

Wai Ping: Does concave mean going in or sticking out?

Mah Ling: It goes in like a cave, that's why it's called concave.

Having previous experience of similar arguments, she hastily retreats to the peace and quiet of her office for the rest of her break.

Granma T enters the room for her break and also gets waylaid by the Wai Ping and Wu Ming.

Wu Ming: Does concave mean going inwards or outwards? In or out?

Granma T swears that she will buy a dictionary next time she is in town because these arguments are getting too frequent. Without answering the question, she too disappears into Mah Ling's office and shuts the door firmly.

Only the children remain to be interrogated and they won't be home from school for a while yet.

Throughout the day the yard echoes to the cousins arguing.

So in the meantime, while waiting for the children to settle the argument once and for all, complete the silhouettes of the two convex shapes that can be made.

Extra activities:

  • Find out what the words 'convex' and 'concave' mean. Can you explain these two ideas to a friend or family member?
  • Make a collection of things that are concave or convex. Can you find anything that's both concave and convex?
  • Investigate when convex or concave lenses and mirrors are used, and find out why they are helpful.

World of Tan continues in World of Tan 14 - Celebrations.