Water naturally absorbs air, and when it freezes, the air comes out of solution, forming bubbles. Ice cubes freeze from the outside in, so the air bubbles, along with any trace amounts of minerals, get pushed to the center as the water freezes, triggering the formation of irregular ice crystals and clouding up your ice.

Commercial ice-making machines are designed to avoid these issues, but you can create sparkling-clear ice at home with no special equipment and a few simple tricks (adapted from a method pioneered by writer Camper English). 

First, use mineral-free distilled water. Second, boil it to drive off much of the dissolved air. Third, insulate the bottom of your ice cube trays, which will cause the ice to freeze from the top down, pushing any remaining dissolved air to the bottom of the cube and leaving the bulk of it clear.

Here’s the method:

1. Line a 13 by 9-inch glass baking dish (glass conducts heat more slowly than metal) with a triple layer of dish towels (each folded in half) and set two silicone ice trays inside it. 
2. Boil distilled water and pour it into the trays, let it cool slightly, and place the setup in the freezer. 
3. Once the water freezes, you’ll find that any cloudiness is confined to the bottom of the ice cubes.