I'm a wheelchair user and was out shopping with dh today. A mum with her young daughter were nearby and the little girl asked in a reasonably loud voice "Mummy, why is that lady using a wheelchair?"
Her mum didn't answer so she asked her another 2 or 3 times. Still no answer.
I glanced over because it's rude to stare and the mum looked incredibly uncomfortable and embarrassed.
I've been in similar situations when I was able bodied and had young children and I'm sure I cringed at their questions....and mumbled a vague response.
Trying to figure out why she didn't answer with a simple "I don't know."
I'm not sure why it's bothering me so much but I feel weird that my wheelchair use and a kids simple question made someone feel so uncomfortable.....it wasn't like she asked a more personal thing like "why does that lady have a moustache." 
How do you deal with these type of questions from your kids?