Creative

Creative mode is one of the main game modes in Minecraft. It is playable in both premium and Classic and singleplayer and multiplayer versions of the game. The Creative game mode allows you to build and destroy blocks in a manner in which you can build any sort of structure you would like.

Players are given an infinite amount of most blocks to build with and no health or damage to worry about. Taking advantage of this, players can create large constructions such as castles, roads, water fountains, and much more along with forms of artwork called pixel art. Not only have users designed buildings, pixel art and caves, but also mini-sports, such as Spleef.

Along with the absence of hunger and health, the player is also able to fly.

Creative Game Play
Once a Singleplayer Creative Mode level is created, it can not be changed to a Survival world (or vice-versa) within the game, but it can be changed by editing the level.dat file inside the saves folder. In multiplayer, players can be individually changed between creative and survival mode with the command available to Operators. 0 signifies Survival, and 1 is Creative. This means that individual players can play creative mode (at the Operator's discretion) on a survival map, or vice-versa.

In Creative the inventory screen is replaced by the Item Selection Screen and shows all blocks and items except Fire, Water, Lava, Mob Spawners, End Portals, layers of snow and Giant Mushroom Blocks. (However, water and lava can be placed by their respective buckets infinitely, the buckets will never empty.)

Dropped items can be picked up by the player, but if the hotbar is already full, the items will go in "invisible" inventory. Even though the regular inventory where these blocks are stored cannot be accessed using any keys, it can still be accessed from the inventory box of a chest, crafting table, furnace, dispenser, enchanting table or brewing stand. Flying is enabled, activated by double tapping to start flying then single pressing  to go up and  to go down and the  keys (default) to move. You can disable flying in mid-air by double-tapping again, causing you to drop to the ground. Touching the ground when flying also disables flying. However, if you get into a minecart while flying, when you get out, you will still be flying.

You can pick blocks into your hotbar by pressing middle-click.

Mobs will still spawn like in survival (including monster spawners), but will not try to attack the player unless provoked. Even then they will do no damage. Creepers are still a hazard, as they might explode near structures.

Normally indestructible blocks like bedrock and portals can also be destroyed. Going too far into the Void will still kill the player, but the player can fly around in the void provided they do not descend below the y coordinate of -64.

Blocks that are destroyed by means other than clicking on them (destroying a block it is attached to) can still be picked up. However, TNT, paintings, minecarts, powered minecarts and storage minecarts can be picked up when destroyed by punching.

Flying very close to the top of a block without touching it will generate footsteps sounds. As flying is way faster than walking, it will generate a lot of footsteps at once. You can also hear wood footstep sounds when hovering right over some non-solid blocks like torches or signs.

If one flies way above the clouds, it will seem as if there are two suns. You are actually seeing the sun and the moon.

Some items and blocks are only available in Creative mode. These are Sponges, Ferns, Chainmail Armor, and Spawner Eggs.

Representation in level.dat
To change the GameType in the  one has to decompress it with something like gunzip (rename it to   first) then open it with a Hexeditor:

You'll find the String "GameType" in clear text followed by 0x000000:

Classic Creative
Minecraft website and clicking 'Classic'.

For obvious reasons, any blocks added in Beta that were not in Classic at the time can not be accessed.

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