Pour a cask-strength whisky, add a drop of water and look through the side of the glass. You will see threads of clearer liquid sinking and curling through the spirit — blenders call them viscimetric whorls. Water is denser than ethanol, so it falls, and as it falls it mixes.
What the water actually does
Flavour compounds that prefer alcohol are pushed towards the surface as the strength drops, so aromas that were locked in at 58% suddenly appear at 48%. Esters bloom first — apple, pear, pineapple — followed by the heavier vanilla and spice notes.
How much to add
Add water a drop at a time and nose the glass after each one. Most malts open between 40% and 46%. Every spirit page on this site has a small calculator that tells you how many millilitres get you there from the bottle strength.
If you cannot see the whorls, you have added too much, too fast.
Watch it happen
The glass at the top of our home page simulates exactly this: move across it and you add water; watch it sink and swirl.