travel mug question

[alex carey]

is this rust and is it bad?

[Eric Kosanovich]

Alex,

It could be rust if you had some type of ferrous metal siting in it, Most likley it is tea or coffee stain. i think you can clean it using vinegar and hot water. Google coffee stain cleaining.You could also try
cleaning coffee or tea stains from cups using denture cleaning tablets.

Good luck
Eric