A scent is a gas that we can detect with our noses. When you smell a flower, you can't see anything, but in fact the flower is releasing gases all the time. These gases are tiny transparent particles too small to see. But those are just the particles that our noses are good at detecting in amongst all of the other particles of gas that make up the air.

If things are nice, we call them scents. If we don't like what we detect, we call it a smell. But it is exactly the same thing, simply gas particles being given off. For example, fresh dog poo gives off particles of gas, but our noses are trained not to like some of that gas, so we think of it as a nasty smell. Of course, other dogs think quite differently and they detect scents given off at the same time which we simply cannot smell. So what is a smell to us is a scent to them!