Teaching why questions
Why asks the child to connect a cause and an effect, which is why it is usually taught after the concrete question words. Start where the reason is visible (it is raining, so he has an umbrella), and only then move to reasons that have to be inferred.
After a correct answer, ask "how do you know?" and have the child point to the clue. That makes the reasoning explicit and is good practice for the How page's level 3 questions too.
How the three levels work
Every question word has three levels, following the usual clinical progression from literal to inferential questions. Level 1 is literal and obvious, with two choices: point to a big thing in the picture. Level 2 is literal but needs looking: a smaller detail, a spatial word, telling two people apart, with three choices. Level 3 is inferential, with three choices: use clues in the picture plus what you know about the world.
Every question was checked against its picture: the right answer is plainly true in the image and every wrong answer plainly false. A session is 10 questions of one type and level, at most two from the same picture.