Displacement: it’s what causes a full bathtub to overflow when you climb in and it’s also what causes your cocktails to creep to the rim of the glass before you take that first satisfying sip. When an object — be it body or ice cube — is placed in a container of liquid, the liquid will move (or displace), making space for the object. It’s kind of like when you plan a dinner party for five people but instead, seven show up — you end up having to shift space at the table to make room for the two extra guests.
So if liquid displacement causes your highball glass to seem more full, you would think that it would overflow when those frosty cubes melt, right? Actually, no. Put a freshly filled ice tray in the freezer and you’ll see why. When water hardens into ice, it also expands; the same is true of most other liquids, but for now we’re focusing on water. For this reason, frozen water takes up more space in your drink than plain water. When the ice melts in any cocktail, instead of having to clean up a spill, you’re merely left with a watered-down drink. No one likes a watered-down drink, so it’s best to drink up before the ice melts.