Make the pudding.
In a pan, stir the gelatin into the water.
Set over medium heat and cook, stirring, until the gelatin granules dissolve and the water is just starting to boil.
Add the mango puree and cook, stirring, just until the mixture starts to boil again. Why not just add the mango puree off the fire? Because the mango puree, at room temperature, will make the gelatin start to harden at once and the mixture will turn lumpy before you can stir it. So, to make the mixture smooth for a uniformly-textured pudding, heat the mango puree too.
Pour the mango-gelatin mixture into a tray. Cool until firm. Cut into small cubes.
Spoon the cubed pudding into glasses. Pour in cold milk. Serve.