A Boston cktail shaker is used to mix drinks with ingredients that are difficult to combine otherwise. They come in two styles - The Two Piece Boston shaker, and Three Piece Cobbler shaker (Manhattan Shaker).
The Boston is made of two tumblers, one glass and the other is usually stainless steel. The glass tumbler typically holds up to 500ml, and the the metal tumbler can hold up to 900ml. When the metal is placed over the glass, they form a seal and make a container for the shaking the ingredients.

To use the Boston shaker measure out the ingredients into the glass tumbler, spirits and liqueurs first, followed by juicers, mixers and any other flavorings. Then add the ice. Then place the metal tumbler over the glass tumbler - give the metal a firm tap with the heel of your hand to seal the two together.
Test the seal by picking the two-part container up by the metal part and, gripping firmly with two hands, shake it well until the chill felt through the metal tumbler begins to become too cold to hold.
Now, holding just the metal tumbler firmly, tap the metal tumbler just below the rim, right about the point where the glass comes in contact with it.
Never bang the shaker against the edge of the bar or other solid object in order to open it as this will often result in just cracking the glass.
Now take your boston cocktail shaker and make a great cocktail
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