In a medium saucepan, combine white vinegar, water, salt, and sugar. Heat over medium until simmering, about 3-5 minutes, then remove from heat and let cool slightly.

Rinse and dry the celery, then cut into 3-4 inch spears. Pack tightly into a quart-sized jar.

Sprinkle red pepper flakes into the jar with the celery, distributing evenly.

Pour the hot brine over the packed celery, ensuring the spears are fully submerged.

Seal the jar and let it cool for 1-2 hours at room temperature, then refrigerate and allow to marinate overnight for best flavor.

Serve straight from the jar or use it as an addition to meals like salads or sandwiches.