Minecraft wiki entry for Nether_portal, associated text: Once a frame is constructed, it is activated by fire placed inside the frame. This creates portal blocks inside the frame, resembling a vortex. The fire can be placed in any manner, including use of flint and steel or a fire charge, the impact of a fireball or small fireball, a lightning strike, or natural spread of fire to flammable material adjacent to the portal. Nether portals can be activated only in the Overworld or the Nether. They cannot be activated in the End and customized dimensions.