[Roy runs a hand through his hair. No point in hiding the truth now.]

When an alchemist attempts to perform human transmutation, they're brought to...some sort of plane and thrown through a giant gate, where they discover...well, the Truth. In the process, they learn how to perform the clapping transmutation that made Fullmetal so famous.

...But they also have to give up something, as per the rules of equivalent exchange. Fullmetal lost his right arm and left leg, and Alphonse lost his entire body.