Layering or “stacking” a drink is the process of gently pouring liquids of different densities on top of each other to create a layered effect. Each liquid has a different specific gravity--it’s own personal weight. Now before we run off and get to scienc-ey, I’ll try and keep this simple. Put the heavy boxes on the bottom and the lighter ones on top. Get it? Good.
Cream liqueurs and fruit juices are packed with sugar thereby increasing their density making them heavier. Obviously those should always be poured first. More alcohol and less sugar makes the liquid lighter and less dense so those layers should go on top. Liquors with the highest alcohol by volume are the lightest and are always floated on top. Each layer should be placed gently atop the next by pouring the alcohol slowly on the back of a bar spoon to avoid upsetting the other layers.
The layering process is also used when making the famous “black and tan” and really shows how ABV and density work. Guinness, which seems like a “heavy” beer actually floats atop the “lighter” Bass Ale because of density. Neat right!