Annoy your coworkers with being a bit vauge about what is in your mug. If they do say anything you can can always say "Well... what do you think?! GOSH! Could be tea. Do you hate the British or something? Whatever. Is there anything else I can do for you today, Jennifer?"

What is the mug? It's a normal glossy black ceramic mug that holds 11 oz or what some other weirdos would say 33 cl. You can drink out of it and everything. Put it in the dishwasher? Sure.