Things can change.  We can build a sandcastle with sand.  A physical change can make something different.  We can change things back to how they are, this is called a physical change.

Ice is frozen water.  Heat ice and it becomes water.  This is called to melt.

Put water in a tray in the freezer.  It will turn into ice.  This is called to freeze.

Water can be heated.  It can turn into a gas, called steam.  Water turns into a gas when it gets hot enough to boil.  Boil is when water is so hot it makes bubbles and some water escapes as water vapor.  Water vapor is tiny droplets of water in the air.  It can be called steam.