Quote fromAnother method is placing sand blocks on top of lava or water, which will raise a lava or water block to the same place as where you placed the sand block. Be careful when raising lava, I once made a lava tower and some lava spilled out causing the lava blocks to spread rapidly in the air and create huge lag. Fortunately this was on a single-player map, and you should always ask the server admin if its okay to build with lava on his map.
Interesting. I can see keeping this out of multiplayer, for obvious reasons, but why does it work in single-player?