A great question. So great, in fact, that it was posed by Aristotle.

Let's stick with the definition of "element" as a "pure substance that contains only one type of atom." This way, we can think about your question in terms of the periodic table. I had a 5th grade teacher who gave us a copy of the periodic table and introduced the concept to us by telling us that we were "holding all of life, and in fact, the universe, in our hand." Pretty cool when you think about it like that.

Anyway, I bet you can figure out the main elements in the human body - what are we made of? First, you can look at any living organism and the elemental composition will be very similar - all living things on Earth are made of the same stuff, basically. In fact, it leads you to that biggest of all questions - how to define "life."

There are six elements that make up about 98% of the mass of any living organism. I'll let you look up the names - here; I'll give you the chemical symbols for the six:

H, C, N, O, P, and S

These are the major "building elements." The C is the main thing - and we have plants to thank for that (they capture and "fix" the C from the atmosphere). And since water is H and O, those two elements are major as well.

After this, we have what are considered the "trace elements" in most living organisms:

Na, Mg, K, Ca, Mn, Fe, Co, Cu, and Zn

You might notice that these trace elements are similar to an ingredient list in a multivitamin supplement - they are the elements that are used to make the "building blocks" work - often, they are involved in helping to regulate metabolism or other enzymatic processes.

There are a few others that you might be able to detect in humans, but these are the key ones.

Our bodies are mostly water, H₂O, and so by total mass we are more oxygen (65%) than anything else.All the most of the rest of our bodies, e.g. skin, organs, and tissues are made mostly from carbon (18%), hydrogen (10%) and some nitrogen (3%). That accounts for 96% of our mass. The other 4% is mostly calcium (1.5%) and phosphorus (1%), and the rest are, potassium (0.35%), sulfur (0.25%), sodium (0.15%), magnesium (0.05%), copper, zinc, selenium, molybdenum, fluorine, chlorine, iodine, manganese, cobalt, iron are usually cofactors for enzymes and present at 0.70% , finally other metals lithium, strontium, aluminum, silicon, lead, vanadium, arsenic, and bromine are present in trace amounts.