Allometric law is a math formula to figure out how two things are related. It's like a measuring tool to compare how two animals, plants, or any living things, change in size over time. For example, if you measure the size of a giraffe's neck, and the size of its body, you can use allometric law to figure out how much the giraffe's neck grows compared to its body. This law can also be used to compare how different animals use their energy, and how much things weigh.