Thermos flask question

[Chickadee]

Hi All

When I heat up the thermos prior to putting soup in it, how long should I leave the boiling water in?

Its been so long since I used the flask that I have forgotten!!

Thanks

[Chickadee]

Thank you redhelen.

[ChuckFickens]

I wouldn't even say that long is needed, it only to take the chill off the inner surface so a few seconds should be fine.

[mccfluff]

1 min 33 seconds exactly or it will implode :-)