When my niece was 2 years old, she was already brave. She was comfortable around livestock, would jump off anything, and try new activities with enthusiasim. She had one fear however: the roomba vaccum. When it would turn on and begin roving about the house, she would instantly start to cry.
When asked about this, it became clear that what was scary about roomba is that she couldn't tell whether it was alive or not, and therefore how to interact with it. Unlike a computer, it made sounds and moved all on its own, but it wasn't fed or petted like her dogs and cats.
She said things like "I don't know if he's alive, but he's still scary."
Other mechanical things like this also scared her, such as the Thomas tank engine that moved around the track on its own after you clicked a button.
Watching kids figure out whether the interactive toys they play with are alive or not is a fascinating exercise, and one adults can learn from as we increasingly use technology, especially computers and phones, as extensions of ourselves.