A poikilothermic organism is an organism whose body temperature changes with the surrounding environment. An example of a poikilothermic organism is a lizard. Lizards cannot regulate their body temperature like humans can, so their body temperature fluctuates with changes in the environment. For example, when it's cold outside, a lizard's body temperature will drop, and when it's hot outside, a lizard's body temperature will rise. This is why you often see lizards basking in the sun on hot rocks or surfaces - they are trying to raise their body temperature.