Obviously, your child is starting a long and brilliant scientific career with this question!
"Boogers" are made up of mucus. Mucus is produced by mucus membranes. Your body has mucus membranes in many places: The stomach, intestines, nose, lungs, eyes, mouth, and urinary tract. All these mucus membranes secrete mucus.
All mucus contains mostly water and mucin (during an infection, mucus also contains dead white blood cells that were working on the infection). The mucin is what makes it sticky.
Mucin is a branched polysaccharide (saccharides are sugar chains). Starch, for example, is a polysaccharide. If you mix corn starch or flour with water, the resulting substance is sticky. Mucin does the same thing. Mucus is essentially a paste made of mucin and water.
Show your 4-year-old how corn starch and water is sticky (when it dries out especially) and he will be looking at synthetic boogers. Fun!
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.