The Cap Assembly is only for the BottleRocket Booster and Capsule bottles. 

The cap does NOT fit either the 18 oz Stainless Steel Water Bottle (bottle with wide-open mouth and cap that is tethered to the bottle) or the 12 oz Sip Spout Bottle.

Designed for cold drinks. Small parts - choking hazard. Not for children under 3 years of age.