Any idea why it is, when I am out with my 18 month old son somewhere, people, especially women, say...?

They're fishing for information and wanting to start a conversation. A man at the park with an 18 m/o baby is usually a single man. If he happens to be married he will reply something like: 'yes he's my son, he's our first child or our 3rd child'. You heard that cute babies are perfect babe magnet eh...:)

ADD: and the answer you're giving them is perfect! It's funny and it gives them the information they need.

Well dont feel bad, my son is very white Im dominican so My skin is darker, one lady asked me how much do I charge his parents to care for him and if I took care of other kids because she was looking for a nanny! some poeple just dont have manners, at least they ask you first with me they just assume Im the nanny, Thank GOD he looks just like my husband otherwise they'll think I cheated on him and that's why the kid is so white

Some people might thing if he is a different color or not, that me may be adopted. Because People may think if he may also be adopted, just because they want a big family, like some other family I know.

I bet they're checking you out for date material. There's nothing cuter than a man playing with a child. Asking if he's yours helps them segue into asking your marital status. I guess you can clarify this by replying to their inquiry with: "Well, that's what my ex-wife tells me, at least."

They might think you look very young, or that maybe your child doesn't look very much like you?

so, you could be watching him for a sister, brother, friend, cousin. what's the problem? Just be proud and say yes he is thank you.

Either they don't see you in him, they don't want to assume, you seem older or younger then they would expect, or they do it to everyone.

They think he is sweet like you,

when people say that to me, I say "I hope so."

probrably because he doesn't look like you.