Eggs (ラッキーのたまご, Lucky Egg), also referred to as Chansey's Eggs, are container and tossable items that appear in Super Smash Bros., Super Smash Bros. Melee, and Super Smash Bros. Ultimate. Their functionality is similar to capsules, but have a few differences from these items.
In Smash 64, eggs can appear via randomly spawning or from the Pokémon Chansey. In the latter case, Chansey can appear from either a Poké Ball and spawn three eggs via Softboiled, or on the Saffron City stage, where it will spawn one egg. The egg's spawning behaviour, however, changed for Melee; while still appearing when Chansey is summoned from a Poké Ball, eggs now only randomly spawn on the game's three Yoshi-related stages (Yoshi's Island, Yoshi's Story, and the past version of Yoshi's Island), as well as on the Trophy Tussle event matches. In Ultimate, eggs only spawn from Chansey in Saffron City, like in Smash 64.
As a throwing item
Eggs are functionally similar to capsules, in that they are both items that contain a single item; both can be picked up and thrown, and both can be broken open via standard attacks. While the egg also has the chance to explode and inflict significant damage and knockback to opponents, the item's standard damage output is considerably less than that of the capsule.
As a recovery item
Eggs can potentially spawn as standard healing items, and their chances of spawning as this can be manipulated with the Item Switch; enabling only food forces all Eggs to be healing items. Disabling recovery items, however, causes all Eggs to be throwing items. Heals 7% damage.
As a stage hazard
A unique form of eggs can be found in Mushroom Kingdom II. The stage hazard of Birdo sometimes spawns on the stage, after which she will sometimes shoot up to six eggs at players. While these eggs deal negligible knockback and damage to players, they become destructive weaponry when air grabbed; almost all of Birdo's eggs explode when thrown, very few of them actually produce items.
Unique to Birdo's Eggs is that they are displayed as two-dimensional sprites, identically to the character's appearance on the stage.
Eggs are basically the same as Capsules, in that they'll release items when you break them open. They've been known to explode, too. The likelihood that certain items may be inside Eggs depends on the type of stage being played. Sometimes Eggs will contain health-replenishing food items; other times, they may hold weapons.