Look at this iron nail. Outside, old metal gates and bike chains often turn a dusty orange-brown. What do you think is happening to the metal, and can that change ever be undone?

Today's big question is how you can tell that a brand-new material has been made. We will test it twice, once fast and once slow. Today we will mix two kitchen materials in a bottle with a balloon on top, set up nails to watch over the coming days, and then sort changes into ones we can undo and ones we cannot. By the end you should be able to say what the two tests have in common.

Some changes can be undone. Melting ice can be frozen again. The orange-brown on an old bike chain does not turn back into shiny metal.

We will mix baking soda with vinegar and catch whatever forms in a balloon. We use a balloon because a gas is invisible: if something fills the balloon, we can see that it is there. We will also put clean nails into three different places, dry air, water and oil, to find out what rusting needs.

What do you think will happen to the balloon, and which nail do you think will change most over the coming days? Say why.

In your group you will mix baking soda and vinegar in a bottle with a balloon stretched over the top, so that anything new the mixture makes is trapped where you can all see it.

The materials, the method and the safety rules are on the card below. Follow them in order, and while it happens watch closely: what do you see, hear and feel? Afterwards, ask yourself whether you could get the dry powder and the vinegar back the way they were.

One decision is yours as a group. Which piece of evidence will you trust first to say that a new material has formed: the fizz, the balloon filling, a smell, or something you can feel on the bottle? Be ready to share that choice and say why you picked it.