Eggs are projectiles which have a variety of uses related to running in Yoshi's Island.
There are many different varieties of eggs and other projectiles:
The basic kind of egg made by Yoshi, or by green egg blocks. They don't have any contents. Upon bouncing off a wall, they become yellow.
These eggs come from a green egg bouncing off a wall, and turn red on the next bounce. They just have a coin inside. Since coins only give you extra lives, they are essentially useless for speedrunning purposes.
Red eggs come from yellow eggs, and have two little stars inside. They don't change color when hitting a wall. Red eggs are a quick way to get stars compared to clouds or rings, but take more planning to be useful.
Each type of egg will normally bounce off of walls up to 3 times. A lick will stop its flight, bobbing it downward in a predictable direction. See Egg Juggling. On the last collision with a wall, or when the egg passes through an enemy such as a bat, the egg flies off the screen.
Level Specific Projectiles
Some projectiles can only be found on certain levels, and thus are only specific to speed running those levels.
Flashing Red Eggs
These eggs have a red coin in them, and are part of the normal 20 red coins for that level. Therefore, they must be used to hit something, like a star cloud or enemy, in order to get all of the red coins on that particular level.. They behave like normal eggs otherwise. Levels containing flashing red eggs: 1-6, 1-7, & 4-E.
Giant Eggs come from eating a large shy guy. They do not fly so much as plummet, turning all enemies on screen into little stars. This is used in certain levels, such as 2-7, to overload the amount of objects the game wants to put on screen, keeping some obstacles from appearing. They can also be used to create additional stars for 100% routes.
These projectiles, resembling baby ducks/chickens, fly in a circle, and try to return to Yoshi. You have to steal them from their mom by assaulting her. If they hit a wall twice, they get scared off and run away. It's possible to manipulate their flight path by dancing around them. Can be used to fly continuous circles around Yoshi, often known as "doing loop-de-loops."