This question, or variations on it, is one of my daughter's favorite questions at this point. "What is wood made of?" "What is glass made of?" You get the idea. What a perfect opportunity to introduce the elements.
Given my daughter's love of cards, The Elements by Theodore Gray is a perfect fit. Of course, many of the elements are unfamiliar and tangible examples don't exist in most people's homes. But, for many of the common elements there are good examples to bring the concept to life.
Among the elements we've found examples of either in pure form or in compounds:
Silicon: glass windows
Iron: kitchen appliances
Carbon: diamond ring, graphite shafts on golf clubs
Titanium: golf club heads, titanium white paint
Copper: deck post caps, pennies, brass statue
Cobalt: cobalt glass
Nickel: nickels and dimes
Gold: pure gold coin, jewelry
Silver: pure silver coin
Sodium: table salt
Chlorine: bleach, table salt
Iodine: table salt
Argon: double glazed windows
Tin: pewter statue
Zinc: brass statue
So, we have fun with element scavenger hunts. We learn about compounds, chemistry, and connections. The best part for me as a philosopher is my daughter's curiosity and persistent questioning:
"What's water made of?"
Hydrogen and oxygen.
"What are hydrogen and oxygen made of?"
"What are atoms made of?"
And so on as far down as we can to or are able to go. It's a great way to begin building a foundation in the basics of chemistry and physics.