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Fall damage is (blocks-3)/2. So a 23 block drop (or more) should kill you (or a mob). But if I'm building a falling trap and want to use water to collect the items dropped, how much higher does it have to be? The floor would have water running over it to gather the items and monsters would fall to that floor. How high a drop do I need?

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You can always store your inventory in chests and "self-test" your mob trap by sacrificing yourself. – Raven Dreamer Jun 1 '11 at 19:49
Why are you only relying on falling? Lava based grinders are proven to work well, and you can put 4 or 5 spawning levels in between. – MBraedley Jun 1 '11 at 21:07
Yeah, lava-based grinders are usually preferred as you don't need a long tunnel for the mobs to fall down through. – Kevin Yap Jun 1 '11 at 21:13
Lava grinders have the chance of burning up some of the drops. – AttackingHobo Jun 1 '11 at 22:22
Not if you build them well. ;) Plus, the mob spawning traps that use lava grinders generally generate so many items that if any were to burn up, the amount would be negligible. – Kevin Yap Jun 1 '11 at 23:33

If you put a fence at the base of the fall, mobs will still take their regular falling damage. Due to the fence taking up 1.5 blocks of space (this is why you can't jump over fences), the mob will hit the fence instead of the water breaking the fall.

Fence blocking fall damage

This mechanic can be applied like this (not my trap - it's part of a world I downloaded):

enter image description here Mobs spawn in this gigantic tower...

enter image description here ...and hit these fences. The water then funnels the items into the middle (which then connects to another stream flowing the lava you can see, to kill any mobs that fell through the middle).

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Excellent trick, +1! – fredley Jun 1 '11 at 20:59
Awesome, I am definitely incorporating this into my next mob trap. – Nicholas1024 Jun 1 '11 at 23:25
I wish I had more upvotes to give for this trick. – Fambida Sep 7 '11 at 6:37

The minecraft wiki page on water states:

[T]he highest number of block you can fall off of without hurting yourself is 3 ... You can hold the jump key to land in water only 1 block deep if necessary.

So considering the inverse, a single block of water will only prevent fall damage if you're holding in the jump key. Since mobs have no reason to jump if there are no players around, a 23-block drop into 1 block of water should be enough to kill any mob instantly. Just make sure you're nowhere near your trap when the mobs fall, so they're not trying to jump as the descend.

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Mobs do jump as they wander around outside the player-detection radius. – SevenSidedDie Jun 1 '11 at 22:06

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