Light travels in straight lines and very fast. When light cannot pass through an object, a shadow is formed on the opposite side of the light source. Objects are divided into transparent, translucent, and opaque depending on how much light can pass through. Some objects produce their own light, like the sun, while others reflect light, like mirrors. Heat transfers from hotter objects to colder objects - when we touch something hot our hands feel hotter and something cold makes our hands feel colder. Materials like metal heat up and cool down faster than insulators like wood.