Pack radishes into a pint sized canning jar and sprinkle the mustard seeds over the top
Combine the vinegar, water, sugar, and salt in a saucepan and bring the mixture to a boil, stirring occasionally
Pour the mixture over the radishes, covering them completely, let the mixture cool to room temperature, and then refrigerate
The radishes will be ready to serve immediately, and will keep in the fridge for about two weeks
Radishes are wonderful served on a piece of toasted baguette with fresh chevre, and topped with the sliced radish greens