Hydro Flask Standard Mouth BPA Free Insulated Water Bottle Sport Cap, Black (Not Leak Proof) Standard Size Sports Cap fits all Standard Mouth Hydro Flask bottles to provide the ideal solution for on-the-go hydration needs The unique internal valve eliminates the whistling associated with most sport caps Hydro Flask Sports Caps are insulated to aide in keeping your Hydro Flask contents cold or hot all day BPA-Free, Lifetime Warranty It never hurts to have an extra cap!