The easiest and lightest way to carry your fluids; Shrinks as you drink to eliminate sloshing and collapses fully to a tiny size when empty; Made with ultra-durable, abrasion resistant TPU material and has Hydrapak's Beyond Lifetime Guarantee; Fits any of Ultimate's UD Vests, Vestas, or Fastpacks; High-flow bite valve auto-seals after each sip, eliminating annoying drips; Can be frozen or filled with hot water (max temperature 60degC / 140degF); Safe and reliable, 100% BPA and PVC free