This document discusses electric circuits and the factors that affect the brightness of a bulb. It explains that the brightness of a bulb depends on the number of batteries used and their arrangement in the circuit. Specifically, it states that a bulb will be brighter with more batteries in series or when batteries are arranged in series rather than parallel. It also notes that bulbs will be brighter when arranged in parallel rather than in series. The document provides examples and diagrams to illustrate these concepts.