Place the ice cubes in a tall, narrow, chilled glass (the cubes should come near the top.) Add the gin, then the tonic water, then the lime juice, stirring well. Garnish with lime wedge, and serve immediately.
Note: To make the ice cubes, simply fill an empty ice cube tray with tonic water, and let the cubes freeze. It takes just a few hours. Covered well, the cubes will remain fresh-tasting in the freezer for at least a few weeks.