Zombies drop rotten flesh, which can be eaten to restore hunger in dire situations.
The below answer is outdated, but I will leave it here for the sake of internet history:
Zombies only drop feathers. No more than 3 at once.
Since they were one of the first mobs created, Notch said that he didn't know what they should drop. Chickens didn't exist at the time, so feathers seemed
like a great idea.
Don't kill zombies for their drops. They're generally considered the least productive
mob to kill for loot. Maybe this will change in the future.
You can download mobs minecraft comes alive
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Put it on peaceful
In Minecraft Classic, mobs can't spawn, so they can't exist naturally. If you play the Survival Test, and when mobs spawn there, you can save it and open it in creative mode so then the mobs can stay there. However, in Minecraft Creative Mode, mobs can spawn, but they can't hurt you because you have no healthbar.
Mobs are enemies, so I suppose that there wouldn't have been.
No, mobs will not spawn in water or ontop of half slabs.
Creepers can not have kids in vanilla Minecraft. No hostile mobs in Minecraft can be bred.
You kill mobs and players
The squid
Ghasts are one of the biggest mobs in minecraft. They look like giant jellyfish.
The Guardian
Grass and stone blocks.