Double-walled and vacuum insulated. That means all the air has been sucked out between the walls and there is nothing to transfer heat or cold to the inside. With the vacuum insulation, the outside environment has an extremely hard time affecting the interior of the bottle and vise versa. KOLD only uses BPA free, premium food grade 18/8 stainless steel...and that's on both walls, using 18/8 stainless steel helps to prevent odor absorption and rust. So choose your liquid of choice and you will always get the true flavor of your drink. The lids are leak-free. So feel confident in throwing your full bottle in your gym bag without your clothes getting wet, or using it as a paperweight in your office. This bottle also comes with 5 different colors of silicone sleeves.