Ah yes, the old "AC leaking water" trick

It is called condensation. I don't know the exact science of it...but the a/c lines get cold, warmer air hits the lines which causes the condensation...which in turn produces water. Same concept if you take an ice cold beer mug out of the fridge & set it on the table. Pour beer out of the fridge (which is warmer) into it and water will begin to drip down the sides of the mug. It's completely normal.