system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some toddlers with early signs of autism point with their thumbs (instead of their index fingers)?
gpt
To "point" you need to isolate one finger.  The thumb is the easiest, because isolation is important to grasping.

But, as any learned behavior, it depends on practice.  Finger painting and many other toddler activities offer opportunities to teach the child to isolate their pointer finger.  This is mostly important to make a proper pencil grip, as opposed to using the "hammer grip" on a crayon or pencil.

Difficulty learning to isolate a finger could be a sign, but then again you have to assure there are many oPportunities to learn this valuable skill.