The core function is maintaining temperature. Look for:
- Double-Wall Vacuum Insulation: The gold standard for keeping drinks hot or cold for extended periods (typically 12-24 hours).
- Sealed Lid Design: A tight-sealing, leak-proof lid is crucial for both temperature retention and preventing spills.
- Material Matters: High-grade 18/8 stainless steel provides superior insulation compared to plastic or single-wall alternatives.
