What is one building material almost everyone has at home, has no added cost to your usual groceries and that's super versatile? Cardboard! Specifically, cereal boxes. Why people throw cardboard from cereal boxes away is beyond me. There are so many possibilities, and this is one of them...

This Instructable will show you how to recreate my own original design for a Secret Opening Box. On the surface, it looks like a simple cube, without any clear way of opening it. But certain parts, when moved in the right order, reveal a compartment inside where you can store your Top Secret Something in safety. Just don't tell anyone how to open the box.

Once I had designed the box, it took me several hours over two days to build, so it's a good weekend project. When measuring all the parts, try to be as precise as possible. Milimetric precision takes some time but produces much better results.

Supplies

Cereal boxes (2 or 3 should do the job)
Scissors
Glue
Pencil
Ruler
Paint (optional - for decorating)

Step 1: Eat Some Cereals

Blueprints for all the parts in the pictures. All measurements are in centimeters.

Your main supply for this project is available in pretty much every supermarket and comes with the added bonus of food inside! When choosing your cereals, forget about taste and check the size fo the boxes. Larger boxes mean more surface to work with (and probably more breakfasts to eat).

Take care of your cardboard. Open the boxes along where they were glued and try not to rip or crumple the cardboard.

Step 2: Draw and Cut Out All Your Pieces

This is the most time-consuming part and it requires a good amount of patience.

Be precise with your measurements and angles. If it's a few millimeters off, the cube will turn out all wonky. The box has four layers, the innermost being marked as the first layer in the blueprints. Each layer is one millimeter wider than the previous one. This is to account for the thickness of the cardboard (two layers of cardboard are about one millimeter thick). Without the extra millimeter, each outer layer wouldn't fit around the inner layer. The final result will be a 7x7x7cm cube.

Make sure all the pieces of the outermost layer (layer 4) are cut out of the same coloured cardboard. This will make the outside entirely uniform. Even if you paint it afterwards, the colour of the cardboard can influence the colour of the paint once it's dry.

Step 3: Glue the 1st and 2nd Layers Together

Stick the second layer of the box around the first, making sure the flaps of the second layer can bend. Glue the back piece of the second layer on to the back of the first. Make sure it all sticks together well. I used pegs but if you're patient you can just hold it all together with your fingers.

Throughout all the following steps, make sure that the different pieces slot together properly. Some parts might need a bit shaving off them to fit better.

Step 4: Glue the 3rd Layer Onto the Box

Basically, repeat the previous step. Only this time you need to stick the thin front part of the third layer onto the flaps at the front of the second layer. This will make the flaps lie straight.

By now you will have noticed that some parts of some layers are longer or shorter than others. This is to create slots where the moving pieces can move in to and keep in place.

Each layer is designed differently so that the weaknesses and cuts are distributed rather than all accumulated in the same place. This makes the box stronger and less likely to unstick or warp.

Step 5: Glue the Layers of the Lid

The lid is the front part of the box. It should slot in to the front of the box snugly.

Stick layers one, two and three together in the right order. While doing this, make sure they all fit nicely in with the front part of the box. If the fit is a bit too tight, shave a little off the edges till it fits better.

Step 6: Glue the Layers of the Key Together

The key is the top right corner of the box. It will lock the lid with the rest of the box, so you can't take the lid off without taking the key out first.

Glue layers one, two and three together, bending the sides at right angles first. The flaps of the third layer should be able to fold nicely over the other two layers at a right angle.

Step 7: Make Adjustments

Now the three main pieces are formed, before you glue the last layer, make sure everything fits together properly. The lid and the key should be able to slot in to the box and each other, forming and almost seamless cube all together.

I had to shave a bit off the top right corner of the lid so the key could fit in properly. I also had to shave a bit off the sides of the key to fit correctly in to the corner of the box.

Step 8: Glue on the Pieces of the Final Layer

Glue on to each side of the cube three little squares, two rectangles and a big square, as shown in the picture. Make sure all the sides of the cube coincide with each other, creating a pattern all around the box. The design of the outer layer will conceal the opening of the box.

When glueing the pieces on to the key and the lid, make sure they fit in with the other pieces on the box (but be careful not to glue the lid, the key and the box all together in the process).

Make the final adjustments and shave off whatever doesn't fit well. Test opening and closing all the pieces to see if it's all working properly.

Step 9: Paint or Decorate to Taste

I left my box raw because I didn't have any paint at the time, but you can get creative and personalize your box. Certain colour schemes could make it even harder to figure out how to open the box, further confusing anyone who attempts it.

Just remember that very watery paints can warp cardboard. Test your paint on a separate piece of cardboard before trying it on your box. It would be a shame to make a mess of things after getting this far!

Try it out and share the results here. If you have any improvements or ways of complicating the design even more, please let me know!