Ice is the second most important ingredient in almost every cocktail you'll make. It outweighs the spirit by volume, and it's the only ingredient that changes the recipe while you're using it. Upgrading from freezer-tray ice has the biggest payoff per dollar.
What ice does
It chills. Cold dampens harsh alcohol and tightens texture. A cocktail at 41°F / 5°C tastes nothing like the same drink at 59°F / 15°C.
It dilutes, on purpose. Classic recipes assume about 20 to 30% water from shaking or stirring (Shake or Stir? has the targets). Skip it and a Manhattan is jet fuel.
It looks good. A clear 2-inch cube is half the visual reason to drink an Old Fashioned at home.
Bad ice ruins drinks. Small, hollow, cloudy cubes have a huge surface-area-to-mass ratio. They melt fast and over-dilute the drink before it's cold.
Maximum surface area, near-instant chill, slushy on purpose
Cracked
Mostly obscure historic recipes
A cube whacked with a bar spoon or muddler; rarely needed
Sphere (2-inch)
Rocks pours, for the look
Same surface-to-volume ratio as a 2-inch cube, so it melts at the same pace; no better drink
A Lewis bag (canvas bag plus wooden mallet) crushes ice in 20 seconds; a rolling pin and clean tea towel work too. Spheres are harder: silicone sphere molds tend to leak and trap air.
Clear ice
Tap water freezes from the outside in, pushing dissolved gases and minerals toward the center, which freezes last. That's the cloudy core. It's harmless, but it looks bad.
Directional freezing fixes it. Insulate every side but the top, and the water freezes top-down, pushing air and impurities into a bottom layer you toss. The cheap version: a small hard-sided cooler of water, lid off, in the freezer for about 24 hours. Dump the still-liquid bottom (or trim off the cloudy bottom inch), then cut 2-inch cubes with a serrated knife.
Do it for rocks pours and photos. Not for shaker ice, which gets pulverized in 8 seconds where nobody sees it.
Filtered or distilled water buys marginally cleaner-tasting ice. Use it if your tap tastes off; otherwise tap is fine. Don't spend money on bottled water for ice.
Pack the tin
Fill the large tin most of the way. Most beginners think less ice means less dilution. It's the opposite.
A packed tin chills fast, and each cube barely melts because it does a tiny share of the work. Three sad cubes melt almost completely in the same time and shed far more water. Same for stirring: fill the mixing glass almost to the rim.
One cube, no refills
Stir a rocks-glass Old Fashioned with one big cube and stop there.
The drink is supposed to change: sharp and aromatic at the first sip, rounder by the third, softer and more open by the last. Topping up resets that arc and dilutes the second half. Same for a Negroni on the rocks or any sipping pour.
Want a fresh, cold drink? Make a fresh, cold drink.
What to buy
Two silicone 2-inch cube trays, $10 to $15 each from Amazon, IKEA, or restaurant supply. Keep frozen cubes in a sealed bag against freezer smells.
Bagged ice from the gas station or grocery store is fine for shaking and stirring. In a rocks pour it'll melt before your third sip.
High-end molds (Tovolo, Wintersmiths) only matter when the ice is on display; skip them for at least your first year. Clear-ice projects, sphere molds, and pebble-ice machines can wait until you've made 100 drinks.