Egg

An Egg is a food item laid by Chickens every 5–10 minutes (it takes 40 minutes for 9 chickens to lay 50 eggs, so 7.2 minutes/egg). They can be stacked together in groups of up to 16, unlike most items.

When thrown, eggs have the same particle effect on impact as snowballs and additionally have a 1/8 chance of spawning a Baby Chicken. There's an additional 1/32 chance of spawning four chickens instead of one. This also works in The Nether and The End.

Eggs are a component of Cake - a single egg is required to craft each one.

Resource farming
Since eggs can be made into cake, and are probably the rarest item in the cake recipe to acquire through normal activities, it can be useful to try and stock up on as many as you can. An easy way to accumulate eggs (without inventory hacking) is to dig a deep pit (preferably at least 3x3, 32 tiles deep). If your pit is deep enough, all non-chicken mobs will die when they stumble into it, thus protecting you from aggressive mobs. If you wait long enough, eventually chickens will fall in of their own accord and start laying eggs. It helps to periodically return to the surface to see if a chicken flock has spawned around your pit - you can (carefully) push them in yourself or hit them with snowballs until they fall in. (Snowballs do not deal damage to mobs)

If the player has already collected several stacks of eggs they can be thrown to manually spawn chickens in a confined area where the player can collect the eggs the flock produces. While a virtually limitless number of chickens can be forced anywhere, the benefit of relatively quick production of a large number of eggs is offset by the number of eggs required to prime such a system due to the low probability of a thrown egg spawning chicken. (probability of a chicken spawning seems to increase as the amount of eggs thrown increases; usually around 20-30 eggs thrown)

Therefore, it may be more efficient to breed chickens using wheat, as two units of wheat will yield a new baby chicken.

History
The Egg was added in the 7 Seecret Friday Update. During the Beta update (20 of December, 2010), eggs were made throwable at the request of a fan as a result of this Twitter conversation.

As of Beta 1.9 Pre-releases, it is rather easy to start a farm with as much as two or even one chickens, as they can reproduce both by breeding and (albeit unreliably) with eggs.

Until Minecraft 1.0 RC1 eggs hatched adult Chickens because baby animals were yet to be added.

Trivia

 * If too many chickens are in a small area they will start clipping into walls, and suffocating and dying. This can be alleviated by using glass walls for a chicken coop, since transparent blocks do not cause suffocation.
 * Like arrows and snowballs, eggs are fired from the dispenser and have the same effects as a thrown Snowball when placed into a dispenser and then activated. Eggs fired in this manner have the same chance of spawning chickens when breaking on the ground.
 * If an egg is thrown through lava, it will catch on fire similarly to an arrow, and will not burn away.
 * If an egg is thrown through water, it will leave a trail of bubbles, much as snowballs and arrows do.
 * Eggs push mobs back, but do not deal damage to them.
 * However, thrown eggs deal damage to the Enderdragon, but since they deal so little damage it is only practical to kill the Enderdragon on creative mode. It is unknown why this happens.
 * If the player is standing near a chicken when it lays an egg, a "pop" can be heard in singleplayer, but not in multiplayer.
 * Throwing eggs at neutral enemies will provoke them.
 * Eggs will break midfall if hit by another projectile; The chance of spawning a baby chicken is not affected.
 * Throwing an egg at a portal will break the egg when it hits the portal, but a spawned chicken will still go through the portal.

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