Cut the lemongrass stalks in half, discarding the green tops. Smash the lemongrass with a rolling pin (or pestle from mortar and pestle) just until the bulbs break open. Combine the smashed lemongrass and water in a small pot over high heat. Bring to a boil and add sugar. Reduce heat and allow to simmer for 10 minutes. Remove from heat and allow tea to cool to room temperature. Serve over ice.