Basic critical thinking suggests to Jim that the super-concentrated espresso (which he definitely pronounces expresso) would be unpalatable without the water, and doesn't question diluting it. He accepts the mug, giving Ianto's proud face a bit of a look. Parts charmed, parts trying not to laugh at how smug he seems over a cup of coffee - though it'd be good natured laughter, because it's cute, somehow.

"Alright," he says, wondering if he's going to have to school his reactions. A practiced caffeine addict, Jim's able to gauge the liquid's temperature between the feel of the cup in his hand and the heat when the thing's half an inch from his face, and wastes no time waffling about trying to cool it further.

Hm.

"It tastes like something," is the grand announcement. "That's sort of unusual for coffee, for me."