NOT bigger on the inside than the outside
Remember that time Rose, as Bad Wolf, opened up the heart of the TARDIS, and the golden steam stuff flowed everywhere and she destroyed the Dalek fleet, and then the Doctor regenerated and became a funny Scottish guy for a couple of seasons? Using this mug is kind of like that. Only, without Rose. Or the Doctor. Or the Daleks. (But we have a mug for that, too!) And steam doesn't usually glow in the dark. But all of the rest of it? Totally almost like using this giant TARDIS styled mug, especially if you put some hot tea or coffee in it and then take it somewhere cold so you can see the steam pouring out of it.
This isn't your Granny's sipping mug. This mug was made to hold large quantities of beverage and to deliver it to you at the right time - your time. Styled to look like the eleventh Doctor's TARDIS, complete with St. John Ambulance badge and white notice on the door, this is the mug all aspiring Time Lords use when they're not satisfying their hunger for fish fingers and custard. While not actually bigger on the inside than the outside, it can hold 16 ounces of your choice steamy beverage. So kick back like a Time Lord and take a drink from your own personal time machine - just don't let the Pandorica open. It's bad for the mug.