Hydro Flask Standard Mouth Flex Straw Cap. Hydration on the go just got easier with our leak proof, insulated Flex Straw Cap. Add it to your favorite Standard Mouth bottle to easily enjoy cold hydration anywhere. Fits all Standard mouth Hydro Flask bottles. Product Features: - Straw cap makes cold refreshment that much easier - Easy to transport; comfortable to carry - Reliably sip and transport your drink without worry - Insulated cap for maximum temperature retention - Dishwasher safe - BPA- free - 2 Straws included - Fits 18 oz, 21 oz, 24 oz Standard Mouth bottles