I found a dungeon with a skeleton spawner, and I'm trying to build a "Death-pit Experience Farm" around it.
With the help of wiki pages and youtube videos, I managed to use water to flush the mobs spawned to a tunnel. At the end of the tunnel, I'm planning to build a water elevator abusing the mob AI, hoping them to swim up 23 blocks and drop down from another pit:
W L W L W L [ room ] W [ with ] L<---<---<---<---<--- [spawner] (L=ladder, W=water, <-- stream)
Everything works fine except the mobs will not swim upwards. They get stuck at the bottom of the water elevator. (If I myself follow the stream and hold "space", I'll reach the bottom of the water elevator and swim up smoothly.) Why aren't skeleton swim up? What have I done wrong?