The plastic-encased stainless steel baby spoon is thermochromic Color Changing Spoon: the "soft bite" plastic spoon changes from green to white when the food or drink comes into contact with exceeds 43ºC, and turns green again when it cools. This is possible because the soft plastic coating (most likely PVC) contains thermochromic additives that make it green. When a spoon is dipped into a hot pot of baby food, the thermochromic pigment in the plastic becomes transparent so that the white color of the plastic base material becomes visible. The bottom of the spoon is metal because it conducts heat well, making the spoon very sensitive to hot substances. It also stores heat, lengthening the plastic's color-changing response to hot food and making it more pronounced.
After doing some research, we learned that the reason for this phenomenon is a process called thermochromic. Thermochromic Color Changing Spoon refers to a material that changes color due to temperature changes. Remember when you were younger and obsessed with mood rings? They use their body temperature to change color. Many companies rely on thermochromic technology to create products that appear to magically change color -- but how does the process work?
There are two thermochromic inks that make these color changes possible: thermochromic liquid crystals (TLCs) and light dyes. According to How Matter Works, "liquid crystal is exactly what its name suggests -- a substance with many of the properties of a liquid, intersecting with the structural elements inherent in a crystal." The properties of thin films vary with the surrounding environment. At lower temperatures, for example, thin films are usually solid, which means they don't reflect much light and appear black.
The next thermochromic ink is a light dye. These dyes are slightly different from thin layer chromatography. First, they have more "long-lasting" chemical reactions. One well-known method is to use light dyes in coors light beer cans.