Tetra means four and this model folds into a 4-sided shape (a
square of course). It's called a 'flexagon' because it's flexible,
so we bend it and move it about, but it always comes back into a
square.
First you will need to draw the six squares or print off this page,
and cut around the shape. If you can, use thin cardboard. (You will
need to 'score' the folding lines by tracing over them heavily with
a pen and using a ruler).
Write the numbers 1, 2 and 3 on the front as shown. Now turn it
over (not upside down though) and write the numbers on the back as
shown below.
Turn to the front again, then fold the two 1's on the top flap
BEHIND, as shown below.
Now fold the 3 that's sticking out, across and tape it to the 2
above it. (See below). You'll know if you've done the folding
correctly because all the 2's will be on the front, and all the 1's
will be on the back. But where have the 3's gone? To make them
appear, gently lift the loose flaps on the front (see where the
dots are on the picture below), and sort of turn it inside
out.
It will be a bit stiff to start with, but it's fun once you get the
idea. If you've used paper, it might tear - but now you know how to
make it, you can make a new one out of cardboard. You might like to
use three colours instead of the numbers.