1 water molecule plus 1 water molecule equals 1 molecule. The molecules combine to form only one bigger molecule. addition in this instance is referring more to mass not units. 1+1=1. So math is wrong when it comes to mass and units. Addition to me means combine. When math was created it was used to count things. Goats, money, bails of wheat. Combining numbers was not understood at the time. When it comes to mass and energy, math needs to looked at in different ways. it works both ways . if you have a large lump of clay and take away 1 piece you have 2 pieces. so 1-1=2. if 3 people get into 1 car you don't have 4 cars or 4 people you have a total of 1 car with a subset of 3.. I call these charged numbers and when the people get out of the car it then becomes neutral back to 1. 1 with a subset of 3. 1(3)+(-3)=1.