A coin has a head (the side with the design) and a tail (the side with the year), but it doesn't have a body.

A fire grows when you feed it with fuel, but it gets smaller when you add water.

The riddle says 'not a single person,' which means they are all married couples.

A yardstick has three feet (the markings used for measuring), but it can't walk.

A shirt has a neck (the opening for the head), a back (the part that covers the back), and four legs (the sleeves), but it doesn't have a head, a spine, or feet.

A candle is tall when it is new, and it gets shorter as it burns.

A shoe has a tongue (the part that goes over the instep), a bed (the insole), and a river (the groove in the sole), but it can't speak, sleep, or hold water.

An umbrella can go up a chimney down because it is pointed, but it can't go down a chimney up because it is too wide.

A clock has a face (the dial), hands (the hour and minute hands), and walks (the second hand), but it doesn't have eyes, arms, or legs.

The weather is always changing, but it always stays the same in the sense that there is always some kind of weather.

The answer is a coin. The equation (x^2 - 4 = 0) has two solutions, (x = 2) and (x = -2), which can be interpreted as the head and tail of a coin.

The air is always there, but we can't see it. It is everywhere, but we can't touch it.

A shoe has a tongue (the part that goes over the instep), a bed (the insole), and a river (the groove in the sole), but it can't speak, sleep, or hold water.

An umbrella can go up a chimney down because it is pointed, but it can't go down a chimney up because it is too wide.

A clock has a face (the dial), hands (the hour and minute hands), and walks (the second hand), but it doesn't have eyes, arms, or legs.