" Fermions are particles which possess half-integer spin and obey the Pauli exclusion principle, which states that no fermions can share the same quantum state. Bosons possess integer spin and do not obey the Pauli exclusion principle . Informally speaking, fermions are particles of matter and bosons are particles that transmit forces. For a detailed description of the differences between fermions and bosons, see the article on identical particles. In the Standard Model, the theory of the electroweak interaction (which describes the weak and electromagnetic interactions) is combined with the theory of quantum chromodynamics. All of these theories are gauge theories, meaning that they model the forces between fermions by coupling them to bosons which mediate (or "carry") the forces. The Lagrangian of each set of mediating bosons is invariant under a transformation called a gauge transformation, so these mediating bosons are referred to as gauge bosons. K Line Model Trains The bosons in the Standard Model are: Unsolved problems in physics: Parameters in the Standard Model: What gives rise to the Standard Model of particle physics? Why do its particle masses and coupling constants possess the values we have measured? Does the Higgs boson predicted by the model really exist? Why are there three generations of particles in the Standard Model? Is the Standard Model reality, a good approximation to reality, or fatally flawed? Photons, which mediate the electromagnetic interaction. W and Z bosons, which mediate the weak nuclear force Eight species of gluons, which mediate the strong nuclear force. "

" In fact, it was so successful that Ford did not purchase any advertising between 1917 and 1923; in total, more than 15 million Model Ts were manufactured, more than any other model of automobile for almost a century. This record has now been surpassed by the Volkswagen Beetle, and the Toyota Corolla and Ford F-Series nameplates have both sold more. The record is currently held by the Volkswagen Golf which sold over 21,000,000 units and is still in production today. It was sold in the beginning at a price of $850 when competing cars often cost $2000-$3000. By the 1920s the price had fallen to $300 (about $3,300 in 2005 inflation-adjusted dollars) because of increasing efficiencies of assembly line technique and volume. On May 27, 1927, Ford Motor Company stopped manufacturing Model T cars. However Model T motors continued to be produced until August 4, 1941. Model Train Stuff Almost 170,000 motors were built after car production ceased. Many Model T parts are still manufactured today, particularly fibreglass replicas of their distinctive bodies, which are popular for T-bucket style hot rods (as immortalized in the Jan and Dean surf music song, "Bucket T", which was later recorded by The Who). "