Les Enfants Terribles

Les Enfants Terribles (The Terrible Children), was a project developed in the early 1970s by Zero and carried out by Dr. Clark that aimed to produce clones of Big Boss, the legendary mercenary and a founding member of The Patriots. The name of the project was presumably inspired by the novel and the 1950s film of the same name. Big Boss was sterilized by exposure to nuclear radiation (Caused during the nuclear testing at Bikini Atoll) 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 identical clones. An egg from Dr. Clark's Japanese assistant was fertilized with Big Boss' DNA and transferred to the womb of EVA, another member of The Patriots, who acted as a surrogate mother. Of the eight embryos created this way, six were aborted, thus producing twin clones. Further work was done so that one clone would express each of Big Boss's dominant soldier genes, which was intended to create a genetically superior soldier, while the other would express his recessive genes, which were considered inferior for the project's goals, and were also given the nicknames of "Brother of Light" and "Brother of Dark." Around 1974, the Patriots (going by the name of Cipher) intended to use the Twin Snakes as their "insurance" in case they failed to either get Big Boss to rejoin them, or frame MSF for a nuclear attack against the United States' East Coast.

The clone with the superior soldier genes became known as Liquid Snake and the clone with the inferior soldier genes known as Solid Snake, although Liquid was led to believe that Solid Snake had the superior genes and likewise he himself had the inferior genes. EVA's reference to Solid Snake as "David" and his prior knowledge of this name suggests that the twins were given full names at birth, possibly by EVA herself. However, Liquid Snake does not acknowledge his given name, and reacts angrily to Solid Snake suggesting he has one himself.

As a result of Big Boss's perceived influence, Liquid believed that he expressed the inferior soldier genes of his father and thus believed that he was the inferior of the original twin clones. In spite of his supposed genetic shortcomings, it was Solid Snake who defeated both Big Boss and Liquid Snake.

The Third Son
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. Thus Solidus Snake (meaning neither solid nor liquid) was created to be an exact clone of Big Boss, without any modifications to gene expression. The egg-donor and surrogate mother used in this phase of the project are unknown. Solidus was later known as George Sears.

Each clone had accelerated aging (or Werner syndromes) written into their genes in order to provide a fail-safe against the clones falling into enemy hands. In Solidus's case, this aging was apparently faster, thus providing the figurehead that The Patriots required (a US 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, President George Sears was referred to as "Solidus Snake". However, he did at one point pose as Solid Snake, and the codename is seen as appropriate considering his lineage.

Trivia

 * Despite being clones of Big Boss, both Solid Snake and Liquid Snake did possess some East Asian DNA in their genomes, as a result of the Les Enfants Terribles project. The donor egg, provided by Dr. Clark's Japanese assistant, would have passed on mitochondrial DNA to the Snakes, though their nuclear DNA would still be entirely composed of Big Boss' genes. It was unknown whether the donor egg used to create Solidus came from the same woman.
 * In the novelization of Metal Gear Solid, President Nixon asked if the birth process of Solid and Liquid was the ninth attempt, to which Dr. Clark confirmed, revealing that the previous eight attempts were failures until they corrected the genetic batch.
 * As Elisa and Ursula died during the San Hieronymo Takeover, she had a precognitive vision of Naked Snake's future: that Big Boss would build his own Metal Gear, one of his sons would bring the world to ruin, and another of his sons would save it. During this state, she explicitly used the phrase "Les Enfants Terribles."
 * In the non-canon game Metal Gear: Ghost Babel, Solid Snake tells Chris Jenner (in an optional codec conversation) that Big Boss was his father, but he does not elaborate on whether it was due to the Les Enfants Terribles project.
 * During Liquid Ocelot's Insurrection, Philanthropy had three chickens that were named after the Les Enfants Terribles children.
 * Solidus was apparently created in secret, as Zero only mentions Solid Snake and Liquid Snake in a phone conversation with Kazuhira Miller.

Behind the Scenes

 * Les Enfants Terribles was also the name of a minor French terrorist group mentioned in the MSX2 version of Metal Gear 2: Solid Snake that the mercenary Running Man originally belonged to. The group is not mentioned by name in the PlayStation 2 version included in Metal Gear Solid 3: Subsistence, most likely to avoid any confusion. The Metal Gear Solid 4 Database, however, implies that both the project and Running Man's terrorist group's names were inspired by Jean Cocteau's Les Enfants Terribles.
 * In Super Smash Bros. Brawl, when Snake fights another player also using Snake, the player can initiate a Codec conversation with Colonel Campbell, who starts wondering, "Has the Les Enfants Terribles project really come this far?" Snake explains that his doppelganger has more than physical and genetic appearances that make them alike, it is also the fact that he has his fighting style as well, although it's not exact.
 * There was an animal (technically a set of three animals) in Metal Gear Solid 3 during the fight against The Boss in Rokovoj Bereg called Les Enfants Terribles. The three individual snakes are called Liquid, Solid, and Solidus.