Here's why:

* Heterogeneous mixtures have components that are not evenly distributed throughout the mixture. You can often see the different parts of the mixture.

* Smoke consists of tiny solid particles of soot, ash, and other combustion byproducts suspended in a gas (usually air). These particles are not evenly spread throughout the gas, making it a heterogeneous mixture.

You can even see the different parts of smoke if you shine a light through it. The solid particles will scatter the light, making the smoke appear cloudy or hazy.