Water flask

The Water Flask is a relic of the Vaults. In Fallout the player is given three flasks if the outdoorsman skill is tagged. At the beginning of Fallout 2 the main character is given a flask from Vault 13, to assist in their search for the GECK. It's worth $25.

A container for the holding and preservation of water or other liquids.

In Fallout 2
In Fallout 2, the player can ask Vic where he got the flask, and heads off to Vault City to see Ed the brahmin dealer. Showing Ed the Vault 13 flask will put the Den, Modoc, Broken Hills, Redding and New Reno on the world map, and give the player 500 XP.

One would think that the Chosen One could show the flask to the 'Alternate Universe-tm' Dogmeat in the Cafe of Broken Dreams and get the same result as revealing the Vault 13 suit, or at least some result, but it doesn't.

Give one water flask to each of the party members, as a symbol of camaraderie, and to weigh each one down by two extra pounds. Yep, there really isn't much to say about the Flask, considering it was a crucial requirement of the Vault Dweller's quest in Fallout, a sacred relic linking the uncertain fate of the starving tribe to the glory days of the past in Fallout 2, and all that.

It weighs 2 pounds.