Les Enfants Terribles

Les Enfants Terribles, which in English means The Terrible Children, was a project initiated by Zero and carried out by Dr. Clark that aimed to produce clones of the legendary solider, and founding member of The Patriots, Big Boss. Big Boss was sterilized by exposure to nuclear radiation (most likely caused during the nuclear testing at Bikini Atoll or by the explosions caused by the Davy Crocketts) and, as a result, natural reproduction was not possible.

The project was apparently considered during the early stages of the formation of The Patriots (it was used by Zero as an incentive for Ocelot to join the organization) but it was not initiated until later on. After a rift began to grow between Zero and Big Boss over the interpretation of The Boss's will, Zero took steps to ensure that The Patriots would retain their figurehead.

For more info on the history of the project see here.

The Twin Snakes
The initial phase of the project involved manipulation of Big Boss' genes to create genetically diverse clones. An egg from Dr. Clark's Japanese assistant was fertilized with strains of Big Boss' DNA and transferred to the womb of another member of The Patriots, EVA, who acted as a surrogate mother. Of the eight embryos created this way, six were aborted, thus producing twin clones, one inheriting Big Boss’ superior soldier genes, and the other his (apparently inferior) genes. They were designated Liquid and Solid respectively. EVA's reference to Solid as 'David' and his prior knowledge of this name suggests that the twins were given full names at birth, possibly by EVA herself.

For an unknown reason, Liquid believed that he had inherited Big Boss' inferior soldier genes and thus believed that he was the inferior of the original twin clones. In spite of his supposed genetic shortcomings, it was Solid who defeated both Big Boss and Liquid.

Solidus
The creation of the original clones was the last straw for Big Boss, who left The Patriots in disgust. As a result, Zero required a perfect clone of Big Boss to provide the DNA-based key to access the master AI (JD). Thus Solidus (meaning neither solid nor liquid) was created with the same perfect balance of dominant and recessive soldier genes that Big Boss possessed. The egg-donor and surrogate mother used in this phase of the project are unknown. Though Solidus was later known as George Sears, it is unknown if this was his birth name or if he even had one.

Each clone had accelerated aging written into their genes in order to provide a fail-safe against the clones falling into enemy hands. In Solidus' case, this aging was apparently faster, thus providing the figurehead that The Patriots required (a United States President in the image of Big Boss) in a relatively short amount of time.

FOXHOUND codenames
When David joined FOXHOUND, he was assigned the codename Solid Snake. Since Big Boss was in charge of the unit at the time, he may have awarded him the codename in reference to his old FOX codename, Naked Snake. Another possibility is that the codename was part of the FOXHOUND ranking system in use at the time, wherein 'Snake' may have represented soldiers with a skill level similar to Big Boss during his time with the FOX unit. A FOXHOUND codename also contained a personal prefix to distinguish between those with the same codename, thus David's clone designation of 'Solid' was used. It is unclear why this was used or which individuals knew the meaning behind the designation.

Later, when Liquid joined FOXHOUND, he was awarded the codename Liquid Snake. It is unclear if the ranking system was still in place (wherein his abilities would assumedly award him the same codename as his brother) or if Liquid chose the codename himself as the other members of FOXHOUND did at that time (he may have already been aware of his relationship to Solid Snake at this time). Having never been a member of FOXHOUND, Solidus was never referred to as 'Solidus Snake'. However, he did at one point pose as Solid Snake, and the codename 'Snake' is often seen as appropriate considering his lineage.

In other games

 * Les Enfants Terribles is a minor French terrorist group mentioned in the MSX2 version of Metal Gear 2: Solid Snake that the boss character Running Man originally belonged to. The group is not mentioned by name in the PlayStation 2 re-release included in Subsistence, presumably to avoid any confusion.
 * In Super Smash Bros. Brawl, when Snake fights another player also using Snake, the player can initiate a Codec conversation with Col. Campbell, who wonders if "the Les Enfants Terribles project really come this far?".
 * Interestingly, when Metal Gear Solid first came out, Liquid Snake mentioned he hated his father for knowingly allowing Liquid to be used to make Solid Snake (apparantly making Liquid the inferior trash), which would have implied that Big Boss had far more in terms of the project than what was said, but the later games reveal that Big Boss was actually unknowingly and unwillingly contributing to the project (as they took his DNA to make the clones while he was in a coma.)