By its nature, water wants to freeze into a clear crystal structure. As it does, it pushes trapped air and any impurities away from it. In an ice cube tray, this process happens on all sides of the cube; the last part to freeze is the center, and that's where the cloudy air-filled ice ends up. To make a perfectly clear ice block without a cloudy core, similar to those that ice specialists like Richard Boccato of New York City's HundredWeight Ice company works with, you simply have to take control of the direction of freezing: Fill a small hard-plastic picnic cooler with water and place it in the freezer with the lid open. The water inside it will freeze from the top down thanks to the cooler's insulated sides and bottom, and only the bottom 25 percent or so will end up cloudy. Let the block freeze completely, turn the cooler over, and wait until the block drops out. Then just score the block where the air bubbles begin, and use an ice pick and a mallet to split the cloudy portion from the rest of the ice. Don't discard it, though; crush it to use in slushy warm-weather cocktails or break it into chunks for your cocktail shaker.