Find the Water Thief.

Find the Water Thief is a side quest in Fallout.

Quick walkthrough

 * 1) Return to Vault 13 after 30 days have passed since the start of the game.
 * 2) Speak to Lyle, Cindy, or the water guard to find out who is stealing the water from the Vault.
 * 3) Wait in the room near the elevator on level 3 until midnight
 * 4) When a lone vault dweller enters the water storage room, wait until he has entered the storeroom and takes some water.
 * 5) Confront the water thief and insist on searching him.
 * 6) Kill the water thief for less XP (can also be done if Charisma checked failed when confronted).
 * 7) Allow the water thief to be killed by a non-companion NPC after starting combat for no XP.

Detailed walkthrough
After 1 day, the player is once again able to enter Vault 13. To receive this quest, the player has to wait until 30 days have passed since the start of the game. They will then need to speak to one of the following people in the Vault to be informed about the water thief:
 * Lyle located on the second level, second room from the left of the screen on the south side.
 * Cindy located on the second level, first room from the right of the screen on the south side.
 * The Water Guard located on level 3, outside the storeroom door.

After speaking to any of these people about the water thief, enter level three and wait there until midnight. A lone vault dweller will leave the elevator and walk around the level before entering the water storage room (even if you are standing right next to them).

When he exits the room the player needs to confront him and insist on searching him (the player must pass a Charisma check, or he will attack). After he is searched, the player will be taken immediately to the Vault 13 officer, who will thank them for their aid and complete the quest.

If the player fail the Charisma check, the water thief will initiate combat with the player. Killing him will net 500 XP instead of the full 1000 XP. If a security officer comes running and kills him instead or an NPC deals the killing blow, you will get nothing. The water thief can also be killed as soon as he emerges from the lift to get the basic 500 XP.