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Discard the ice water when you are ready to serve your drink. Fill the glass 2/3 to 3/4 full of ice. The cooling of the ice in the mixing glass will create a vacuum that will help to hold the seal between the two containers.  You may need to gently strike the outside of the tumbler where the metal meets the glass with your hand to break the seal. Set the mixing tumbler aside.

Summary:
Chill a serving glass by placing it in the freezer for 5 minutes or by filling it with ice water. Pour the ingredients for your cocktail into the mixing glass. Invert the tumbler and place it over the top of the mixing glass, pushing tightly to seal the tumbler against the mixing glass. Shake the cocktail over your shoulder using one hand to hold each container. Hold the mixing glass in your non-dominant hand and use your dominant hand to remove the mixing tumbler. Place a hawthorn strainer over the top of the mixing glass and strain the shaken cocktail into your chilled glass.