Very strange question indeed Kazz,
I think my quizzical mind would have had to ask them why? Do they have a dog which does not use 4 legs? :-D I think I would have had to question the question as it's a riddle and needed explanation, there must have been a reason for it............. you'll have to be left wondering.
I've met many a strange person on a dog walk, I'm always friendly but like you I attract the nutters and strange question sometimes and I'll never forget the woman who attached herself to me all I did was say hello, and for the next 60 mins she cloaked my every move and boy did I try to shake her, I had her in my ear telling me about her past dogs ailments then her own bowel problems (and textures

) and that she pee's in the woods and all sorts of inappropriate things to tell a complete stranger and believe me I was not looking interested. In the end I made my excuses and said I had to call in on a friend and cut across to my local farmers house for a chat to shake her.
When my mum was ill and I walked her pack too I even had this couple who regularly would stop me and ask for one of the dogs saying it would go well with their's, everytime they saw me they would ask for the dog as though I were some walking puppy farmer, made no difference how many times I said the dogs weren't for sale they still continued to ask. So yes, plenty of aliens around...................