Oh yes! DD5.5 started asking 'why' questions when she was 3 and she makes me feel completely stupid and ignorant...

Some examples: why do birds do not have teeth? why ducks float in the water?

Then she asked a lot about death which I usually gave very vague answers. Despite this at four she already understood that everybody dies of mostly old age, but as well of illness or injury and that this includes herself.

Most of the time she is busy with science questions. She wants to know about photosynthesis, of what is made the sand, the bread, etc.