I am running some new players thru the "Scales of War" and one of my players is playing an illusionist wizard, with Summon Shadow Serpent as his spellbook 2nd lvl utility.
He had selected that one for the day and when encountering a door, after having them beaten low on resources in room 13 (Von Jallich crypt; arcance sigils on floor) they used the serpent to spy out then next room (14). Inside this room was a fountain of cool water and a prisoner. They explored the room for a bit and when it entered the fountain, I informed him that the snake felt sleepy, yawned and closed my eyes. His summon was then dismissed due to time.
They were freaking out about how they fountain must be poisoned or enchanted. They entered the room to rescue the prisoner, and were acting worried about the fountain. The wizard used arcana to try and determine what was up with the fountain, but failed to detect anything. Finally, after wondering outloud about what caused the snake to fall asleep (I had only said sleepy) I gave the cleric (a follower of Corellon & trained in nature) to make a nature check to realize.
Initially, she recieved a reminder that snakes are cold-blooded, then explained what cold did to snakes, and finally explained it to her. The fountain was filled with "cool and pure fed water." We all thoroughly enjoyed the experience, it has become one of their first favorite moments.