To make a model showing how cubes are made up of three square based pyramids, you will need a cardboard box, some string, some colouring pens and a knife or scissors to cut the box.
From one corner of a box cut squares all exactly the same size to form 3 faces of a cube, and colour the squares as in A.
Repeat this and the two pieces together make a cube.
Use one of these pieces and imagine 3 square based pyramids as shown in 3 different colours in C, D and E. These pyramids fit together to form the cube.
The 7 black lines in B represent strings joining the top vertex to the other 7 vertices of the cube. If the strings are the right lengths they form the sloping edges of the 3 square based pyramids.
Imagine you are looking down on the cube in view B from above. You will see that you can hang the model so that the 3 card faces are at the bottom. If you need help to find the correct lengths for the strings see https://aiminghigh.aimssec.ac.za/grades-8-to-12-cuboids-and-roots