1. In a saucepan, combine rice vinegar, sugar, salt, and red pepper flakes. Bring to a boil, then reduce heat and simmer until sugar and salt are dissolved.
2. Place watermelon rind strips in a jar.
3. Pour the hot vinegar mixture over the watermelon rinds. Let it cool to room temperature.
4. Seal the jar and refrigerate for at least 24 hours before serving.