I agree that's what OP is saying, I'm saying that's not true. The "is" operator here isn't commutative, "tap water is desalinated water" is true, but "desalinat

I agree that's what OP is saying, I'm saying that's not true. The "is" operator here isn't commutative, "tap water is desalinated water" is true, but "desalinated water is tap water" is not. Let's put some numbers to it. Assume tap water is desalinated, it costs $1 to desalinate a unit of water, and $1 to deliver a unit of water to taps. Then tap water is going to cost $2 while desalinated water only costs $1. Delivery to taps isn't free, so if you just want to buy bulk desalinated water it's cheaper.