* Everything in the universe is made up of substances. Whether it's a rock, a human, or a star, they're all composed of different elements and compounds.

* The percentage of substances can vary wildly. For example, the percentage of water in a human body can fluctuate depending on hydration levels. The percentage of iron in a meteorite will differ from the iron content in a rock on Earth.

So, to answer your question directly: Nothing always contains the same percentages of various substances.

Here are some examples of things where the percentages of substances can change:

* Mixtures: Salt water, air, soil, and even a glass of milk are mixtures where the components can vary.

* Solutions: A sugar solution can be more or less concentrated depending on the amount of sugar added.

* Living organisms: The composition of our bodies changes as we eat, drink, and even breathe.

* Geological formations: The mineral content of rocks and soil can differ greatly depending on their location and age.

Let me know if you have any other questions!