Why sugared cucumber might taste like melon

This week, expiry dates on vaccines: what happens if I get a Covid-19 jab that's expired? Also, chemically-speaking what is a smell and how does the body's olfactory (smelling) system work? Why does the smell of fruits change during the ripening process? And why might sugar on cucumber taste like melon? Do some people inherit genes that mean some smells are inapparent to them? When clouds disappear, where have they gone? Why does my nose run when I walk? How does a cellphone touchscreen work? And why do car exhausts seem to last longer these days?