[DISCUSSIONS] Why Does The Lens on Z2 always moist up?

So let's start the discussions with Hi :). Im pretty sure many of the Z2 here love to take pictures underwater.But some of the models give moisture on the lens and the PHONE IS 100% IN PERFECT CONDITION. after the action underwater.So why does this happens even when the phone on normal temperature before dipping it under water(Tap water for example). Can anyone explain?