How do you explain smoking to toddlers/ young children?

Hi all,
Today I was taking DS into swimming and we walked past a man smoking outside the swimming place (idiot I know!) and DS was very curious as to what the man was doing. He kept staring and asked me what's that man eating? I didn't know what to say, I was a little stumped so changed the subject. Dp and I don't smoke, a few family members and friends do but generally not in view of DS. I'm curious as to how people explain smoking to their kids? DS is only 2.5 and I just don't know how to say it to him? Do I just say yukky? What have you said to your toddlers/young children explaining smoking?

Tell him the truth, my theory is if u instil it in their heads early they might be put off smoking later in life. My brother smokes outside so we had to explain why he was going outside. We explained what he is doing, how much it smells and it makes you very sick.

Gotta love when kids ask u the questions u would prefer to avoid!