4 Tips to Know Before Buying 1. This mug doesn't fit standard car cup holders. If that's a must for you, we recommend choosing a suitable size. 2. Always lock the lid when driving to prevent spills on bumpy roads. 3. Cold liquids are fine, but heating time depends on the starting temperature and desired warmth. 4. Not dishwasher-safe. Handwash only, and don't submerge the mug—the battery at the bottom isn't waterproof.