Substances
The things around us are objects, and what they are made of is a substance. For example, a glass is an object and glass is a substance; a spoon is an object and iron is a substance. Substances have three states: solid (keeps its shape: stone, iron), liquid (takes the shape of its container, volume stays the same: water, milk) and gas (keeps neither shape nor volume: air, vapour). In nature water appears in three states: ice, water and water vapour. At 0 °C water freezes and ice melts; at 100 °C water boils and turns into vapour, and when vapour cools it becomes water again (condensation). Water expands when it freezes, so a full bottle left in the freezer can burst and water pipes may crack in winter. Other substances also melt when heated, but at much higher temperatures: glass softens at several hundred degrees, and iron melts at about 1500 degrees.
Experiment: put ice cubes in a bowl and leave them in the room. Watch every 10 minutes as the ice melts into water. Then see steam from hot tea turn into droplets on a cold spoon (with an adult). Which changes of state did you see?