In this unit, students gather evidence to help them explain how light can help animals, including humans, survive. They begin by considering how we can see things in dark places such as caves. They use a computer simulation to investigate whether or not certain objects are light sources. Students collaboratively plan and carry out a series of investigations about how a beam of light interacts with transparent, translucent, and opaque materials. As students form explanations of what they observed, they describe the formation of shadows and reflection of light as effects of light shining on specific materials. They read about the structures of some animals that take advantage of light to help them survive, and also read about the structures that make up lighthouses and the engineering problems that lighthouses are designed to solve.