Leave iron in air and water, and it turns to rust. Rust is no longer iron. It is a new kind of matter. Iron makes it with oxygen and water. Rust is something new.

Mixing can make something new too. Bubbles of gas can give it away. Cooks use this in batter. Lemon juice is sour, and baking soda reacts with it. The gas bubbles make the batter rise.

Some mixes do not stay mixed. Shake oil and water, and they mix for a short time. Then they separate again. Mayonnaise is different. It is oil and water, held together by egg yolk.

Egg yolk holds lecithin, a molecule. One end likes water. The other end likes oil. So lecithin holds tiny drops of oil in water. That is how mayonnaise stays together.

Lemon juice and other citrus fruits taste sour. The reason is the acids in them. Vinegar has an acid too. Red cabbage juice has one color in an acid. A base gives it a different color. So it is a natural indicator.

Red cabbage is not the only one. Berries and beetroot can make an indicator. Many other plants can too. With a grown-up, you could try berry juice and a little vinegar in the kitchen. Look for a color.

Words to know

• rust: iron's red flakes
• matter: what things are
• lecithin: egg yolk part
• indicator: a color clue