How to clean a thermos (without scrubbing)

Your thermos keeps coffee hot for hours. It also keeps a brown ring at the bottom for months. Here's how to fix that in a few minutes, no brush gymnastics required.

Why a thermos gets gross

Coffee and tea leave tannins and oils that stick to stainless steel. The narrow neck makes it hard to reach the bottom, and the lid traps moisture under the seal. Rinsing alone doesn't cut it.

What you need

• Warm water (not boiling)
• 1–3 fizzing tablets, depending on size
• A cup for the lid and seal
• Optional: a seal brush for the grooves

Step by step

1. Fill the flask with warm water nearly to the top.
2. Drop in tablets: up to 500 ml one, 500 ml–1 l two, over 1 l three.
3. Leave the lid off while it fizzes. Never close it during fizzing.
4. Soak the lid, seal and any straw in a cup of the same solution.
5. Wait 15–30 minutes. For old stains, leave it longer or repeat.
6. Rinse everything well with clean water. Don't drink the solution.

How often?

For a flask you use daily, once a week is plenty. Coffee, cocoa or smoothies? Go a bit more often.

Quick tips

• Air-dry with the lid off so moisture doesn't get trapped.
• Take the seal out now and then — that's where slime likes to hide.
• Don't use the tablet on uncoated aluminium, wood, bamboo or cork.