An example would be the reaction between simple metal Potassium (K) and Water (H2O), resulting in a much solid compound called Potassium Hydroxide (KOH) and hydrogen gas is set free.
take a jug of water by finding the volume of that water and ask the 2 year old to chug it all in... the volume of water drank by the 2 year old is the volume of the two year old