You're thinking about sodium metal which like lithium is highly water reactive and is stored in oil to prevent contact with water. Many years ago I worked as a chemist in a hazardous waste disposal facility near Vancouver, BC and found a jar with what looked like small, whole walnuts in a liquid. Turned out it was small sodium chunks that had slowly reacted with moisture over 50 years. Cut one open and there was a pea sized piece in the middle.
A few of the guys I worked with and I used to walk the half block to the Fraser River at lunch to smoke some pot and goof around so I took the jar and some tongs with me. I chucked the open piece in the water where it popped and burned while skittering around on the surface on fire. When whole pieces were tossed in they sank a few feet before water got to the metal then they exploded like small hand grenades sending a column of water 10 - 20ft in the air. Fun was had by all!
I still have a 1lb piece of sodium sealed in a tin can and a jar with lithium chunks in oil that must be 1/4lb or so of the metal. What to do with that I wonder.
