This is a simple but very very loud banger that I used to make as a child. It's easy to make from ordinary household materials and produces an incredibly loud, resounding bang, similar to bursting an inflated paper bag.
It can be reused several times - until the paper is eventually torn
Do not creep up behind people and make a bang - I am not responsible for your actions and the consequences.

You will need a thinnish cardboard
Scissors
Strong brown paper
Glue
Pencil
Ruler

Step 1: Step 1

Take the cardboard - I used a thin corrugated cardboard about 2mm thick. But I think any medium thickness card will do. Cut it into a square - mine was 34 cm x 34 cm. Size isn't critical and I assume that the bigger they are the bigger the bang.
Mark out the two diagonals with a pen or pencil
Fold the card along one diagonal - try to get a neat clean fold.

Step 2: Step 2

Mark and cut a paper triangle in the brown paper. The triangle must be able to cover the upper half of the cardboard and have an extra 25 mm added to the sides to allow for folding over and gluing.
Place the paper triangle in position along the marked (unfolded diagonal) and fold the edges of the paper back over the cardboard and glue in place to the cardboard on the other side.

Step 3: Step 3

Let the glue dry.
Then fold the cardboard square along the original fold. You will also fold the brown paper as well.
Hold the cardboard so that the pointed end of the triangle faces down and chop the whole thing down sharply - as if you were chopping something with a knife.
All being well the downward motion will push air into teh brown paper pocket - which will fly out with a very loud BANG.
You can then reload - by refolding and it's ready to go again.
Have fun but don't scare your grandmother