Preparing a drink, however complicated, is a mixture of art and science. The art bit is easy. The magic is when seemingly unconnected liquids combine to make something that tastes and sounds much better than the parts used in the creation process. Hence neat vodka and humble orange juice combining to make a Screwdriver. Coke plus rum equals Cuba Libre. It still never fails to impress us.
However, get your parts mixed up and it's a very different story. A screwed-up screwdriver tastes similar to Lilt with a dash of lighter fuel while it's easy to make something not dissimilar to fizzy petrol when the rum/cola equation goes awry. Lead crystal glass or plastic Star Wars Pepsi cup from Warner Village - the vessel is irrelevant, getting the measures right is imperative.
Which is where the Jigger Measure comes in. Handily made so that 25 ml (at one end, and 50ml at the other end) of booze fits into it, it guarantees that you get exactly one or two UK measures of your chosen tipple. Grandad can't whinge about his 'special tea' being wrong and you'll be thanked for the best sex on the beach that anyone's ever had.
More detail and specification