Any tumbler must be robust enough to endure the journey. Our Rambler® 591 ml / 20 oz is made from durable stainless steel with double-wall vacuum insulation to protect your hot or cold beverage at all costs. While the magnet on the included MagSlider™ Lid adds an additional barrier of protection for keeping drinks contained and preventing heat or cold from escaping, please note - this magnet component is not leakproof and will not prevent spills.