The second law of thermodynamics says that the entropy (a measure of disorder) of the universe is always increasing. Yet living organisms exist. Why does this not violate the second law of thermodynamics?

Entropy is a fundamental concept in the second law of thermodynamics. It measures the level of disorder or randomness in a system. Whenever a system undergoes a process, its entropy can either increase or stay the same. Keep in mind, entropy never decreases in an isolated system. More disorder means higher entropy.
For example, when ice melts into water, the molecules become more disordered, leading to increased entropy. Always remember: higher organization corresponds to lower entropy.

An isolated system is one that does not exchange energy or matter with its surroundings. The universe can be viewed as an isolated system, as nothing can enter or leave it.
In isolated systems, entropy can only stay the same or increase. An example is a sealed thermos bottle that keeps coffee hot. No matter or energy can escape or enter, making it isolated.

Open systems, unlike isolated systems, can exchange both energy and matter with their surroundings. This characteristic allows them to maintain or decrease local entropy by taking in energy or matter.
A good example of an open system is a human body. We take in food (matter) and oxygen (energy), which helps us build complex structures like cells and tissues, effectively reducing our internal entropy. However, as we do so, we generate waste and heat, increasing the entropy of our environment.

Living organisms are prime examples of open systems. To maintain their structure and function, they consume energy and matter from their surroundings. This local reduction in entropy helps them grow, reproduce, and repair themselves.
Even though organisms decrease their internal entropy, they simultaneously increase the entropy in their environment. For instance, plants take in carbon dioxide and sunlight to create glucose, but they also release oxygen and heat, contributing to the overall increase in entropy in the universe. Thus, living organisms do not violate the second law of thermodynamics. Instead, they illustrate the balance between local entropy reduction and overall entropy increase.