The bilingual preschooler Juli (age 3;7) began to ask "why" questions in American Sign Language.
First, I tossed something in the sink and Juli asked me why you throw ix-ref. Later that same week, I returned my candle holder to its original place after she fidgeted with it. She again asked why you put-it-there in ASL.
Two "whys" during that week suggested the emergence of asking a why question. Asking why questions is a normal developmental milestone for 3- and 4-year-old preschoolers.
In the next weeks, Juli asked questions using "why" increasingly. For examples. One day she noticed a different sarong covering the chair. She noted ix new. I nodded. She asked Why ix-ref.
Another time, she asked me, why you put remote-control up in-there, why?. No need to answer her question. She knew why.
In the meanwhile, she also used something/someone sometimes in parallel.
A few weeks later (age 3;7), Juli asked for a band-aid and struggled to open it. She asked how open ix(this) band-aid? I showed her. She replied, oh!
Juli (3;2) had asked me a yes/no question many times, even though her questions are a statement because she hadn't developed facial grammar or facial intonation to indicate a question. I instinctively understood her.
But, one day I decided to encourage her to use a wiggling question, which is an ASL sign for a question marker which can be accompanied with a required facial grammar for question. Whenever Juli asked me a question, I asked her to use the ASL sign "question".
Shortly, Juli did use the wiggling question one day when she asked me a yes/no question.
Some of the following examples that Juli had uttered during this month in August: careful, keep and some more.
She insisted me to let her touch the glass ball and tried to assure me, ix-me very-careful ix-ref(glass).
Juli used "about" in her sentence, ix-me sad about father.
One morning, she and I talked about fire safety. I asked her, if there were fire in this house, what would you do? I explained that she must inform me immediately. She asked a conditional question, if fire in school, do-do? I replied, Tell teacher really-fast.
Also see Age 1: answering wh-questions