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For hot drinks:

• Start with a clean thermos. 
•  Warm your thermos by filling it with boiling water. Put the lid on. Let it sit for a few minutes then pour out the water.
• Once your thermos is warm, quickly add boiling hot drinks and put the lid on tightly.

For cold drinks: 

• Start with a clean thermos. 
• Chill your thermos in the fridge or freezer. 
• You can also fill the thermos with ice and water. Put the lid on. Let it sit for a few minutes to cool then pour out the water. 
• Once the thermos is chilled, quickly add cold drink (at or below 4°C or 40°F) and put the lid on tightly.