Reasoning is the ability to arrive at the best and most predictable outcome. Reasoning skills are important for logical thought and decision making.
Examples of Reasoning at various ages:
A baby wants to see her father smile, so she puts a blanket over her head, then pulls it off. Each time she pulls it off, her father says with a smile, "Peekaboo!", just as she predicted he would.
A toddler sees his mother cry during a sad TV show. He knows that whenever she crys, his mother kisses him to make him feel better, so he decides to go to his mother and kisses her on the cheek to make her feel better.
A preschooler is outside playing when she sees dark clouds in the sky and feels the wind start to blow. Her previous experiences with dark clouds and blowing winds have led to rain, so she thinks that it will soon rain on her. Not wanting to get wet, she decides to go inside the house.
A high school student is studying at home when the phone rings. It is a friend who wants to talk. The student knows that this friend will talk for hours, and he also knows that he has a test the next day that he really needs to study for. He reasons that he will probably do poorly on the test if he doesn't study, he politely ends the call with his friend and goes back to his schoolwork.