A magnifying glass starts a fire by localizing photons through the pin point area that is created by the convex shape of the lens. To work, sun rays must pass through a magnifying glass of the appropriate focal strength and rest on dry kindling or material that is easily ignitable.
Photons are particles that transmit light and energy in the form of heat. The unique convex shape of a magnifying glass pulls the photons to one side of the lens, localizing them into a single point of exit. This creates an extremely high heat concentration, of about 450 degrees, that is capable of starting a fire. The larger the lens is, the easier it is to start a fire. However, even a pocket magnifying glass is capable of starting a fire.
Any convex lens, such as those found in eyeglasses or binocular lenses, can be used in the same way. A drop of water placed on the lens can intensify the sunlight's heat, causing the fire to ignite more quickly. Glass bottle bottoms are another item that can be used and discarded glass bottles are often a precursor to forest fires. This is one reason why it is important never to leave glass lying around outdoors.