Home-made ice freezes from the outside in. Air that is dissolved in the water, plus any minerals (which is worse if you have hard water) are pushed to the center, last to freeze, where they create bubbles and haze. The resulting ice cubes contain less actual ice than bubble-free ones of equal size would, so they melt faster. And, when they melt, you may find a sediment at the bottom of your glass (this is the previously-dissolved minerals). To avoid the haze and sediment, use distilled water (which has no dissolved minerals) and bring the water to a boil briefly, then cool and freeze as usual. The boiling drives out most of the dissolved air, so no bubbles. You'll get clear, sediment-free cubes that last longer. Worth the effort? Maybe only for special occasions!