Hammerspace

Hammerspace is a term used to refer to an invisible space where fictional characters can pull out objects larger than the physical space they are stored in. RPGs (Role-Playing Games) refer to this space as a Magic Satchel. In the Sonic continuity, many characters are seen pulling large objects out of nowhere (usually >Chaos Emeralds). This can be considered as sort of a "video game logic" that has no real meaning to it and is not important to the story or gameplay, but can sometimes happen as an element of gameplay.

Users
Many characters have been seen having access to Hammerspace, such as:
 * Sonic the Hedgehog
 * Shadow the Hedgehog
 * Silver the Hedgehog
 * Amy Rose
 * Knuckles the Echidna
 * Miles "Tails" Prower
 * Dr. Eggman
 * Espio the Chameleon
 * Vector the Crocodile
 * Rouge the Bat
 * Bean the Dynamite
 * Chip

Usage in Games
In games, characters are commonly seen pulling objects out of nowhere. This includes, but not limited to the following.

Sonic Heroes
In the last story, after Eggman shouts about the Chaos Emeralds, several characters all pull out a Chaos Emerald, all seven in total, completely out of Hammerspace. Also, during gameplay, a few objects are pulled out of hammerspace by a few characters.

Amy Rose
Amy can pull out impossible amounts of Piko Piko Hammers out of hammerspace. This is even parodied in Sonic X where Amy simply holds out her hand a hammer appears in a puff of smoke. And in another episode, it appears in a flash of light.

Espio the Chameleon
Espio can pull out an indefinite amount of ninja stars out of hammerspace.

Big the Cat
Big can pull his fishing rod out of hammerspace to attack enemies.

Vector the Crocodile
Vector can pull out an infinite amount of bubble gums to blow bubbles with (Team Chaotix version the Triangle Dive).

Sonic Battle
Several characters can pull objects out of Hammerspace when performing special attacks:
 * Sonic can pull out Rings and explosives
 * Tails can pull out boxing gloves, energy cannons, explosives and magic hands
 * Knuckles can pull giant boulders out of the ground can also pull out explosives
 * Amy can pull out Piko Piko Hammers, explosives and boxing gloves
 * Rouge can pull out large balloon shaped objects
 * Cream can pull out spade-like objects
 * E-102 Chaos Gamma's Jet Booster will appear when activated and will disappear when Gamma hits the ground
 * Emerl and Phi can pull out all of the above.

Sonic Advance 3
After Sonic defeats the the final boss, Gemerl tries recomposing himself from the damage from this battle, but then suddenly flies at Sonic and makes the Chaos Emeralds appear out of nowhere.

Shadow the Hedgehog (game)
In Shadow the Hedgehog, Shadow was shown having access to Hammerspace. He would often pull any Chaos Emeralds he had collected out of nowhere and then they would just vanish without any physical means of transporting them. Although in one scene he made a gesture where he appeared to be putting two Chaos Emeralds in his "pockets" although he wears no clothing.

Also, the guns Shadow carries also use hammerspace, indicated by the fact that he never needs to reload them.

Sonic the Hedgehog(2006)
Several characters pull Chaos Emeralds out of nowhere.

Sonic Chronicles: The Dark Brotherhood
In Sonic Chronicles: The Dark Brotherhood, the player is capable of collecting an indefinite amount of items, without any visible means of transporting them.

Sonic Unleashed
Chip is able to pull chocolate bars out of hammerspace.