Press esc and the Game Menu will pop up. Press options, and check if you are on Peaceful mode. if you are, press that Peaceful button, and it'll turn to Easy. Keep pressing until you have the right game mode that you want. Peaceful -> Easy -> Normal -> Hard
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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.
You can download mobs minecraft comes alive
No, mobs will not spawn in water or ontop of half slabs.
Ghasts are one of the biggest mobs in minecraft. They look like giant jellyfish.
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