Swirl water in a glass and pour out excess. Freeze glass until frost can be seen. Add ice to a tumbler. Pour in vodka and gin (use Absolut and Tanqueray). Shake tumbler until mixture is extremely cold. Pour the gin and vodka mix into iced martini glass and add Grand Marnier (about 2 capfuls). Twist an orange peel over the glass and drop in the sliver. Serve very cold.