Condensation

Condensation was the process by which liquid formed from gas.

Its happening on the walls and ceilings inside the Doctor's TARDIS was explained by Ian Chesterton as occurring when "the [TARDIS] cooled, but the inside stayed warm — and so moisture formed on the inside". This sudden appearance of water greatly benefited Marco Polo's party, as they had been rationing water for some days while journeying towards an oasis. (TV: "Five Hundred Eyes")