If you want the chemical explanation....
Magnesium sulfate attracts water molecules. Seven water molecules, specifically. In the US, we ignore them, and just call it "magnesium sulfate."
But in Canada (and other locations in the world, such as the U.K.), it's spelled "sulphate"... And you acknowledge the seven water molecules that are attached to the molecule:
- Seven = hepta
- Water = hydrate
Which gives "magnesium sulphate heptahydrate" aka "Epsom salt."