Things can change.  Eggs, milk, flour and butter can make a cake.  We bake a cake in an oven.  Once the ingredients are made into a cake, they cannot go back to being a cake.  Wood can be burned in a fire.  The wood becomes ash and smoke.  The ash and smoke cannot go back to being wood.  This is a chemical change.

Things can burn.  They can burn with a fire.  A fire uses 

Things can change with a fire.  Wood can become ash and smoke.  This is called to burn.

We can put things together.  This is called to combine.  Combine means to mix things together.