Thumb facts for kids
The thumb is one of the five fingers on the human hand. Thumbs are different from the other four fingers because they are in a different direction, have only two bones (the others have three) and are shorter. Thumbs help us to pick things up. This is called having "opposable thumbs". All primates have thumbs but humans have fully opposable thumbs.
Images for kids
A bonobo "fishing" for termites, an example of incomplete/"untrue" opposition
Thumb Facts for Kids. Kiddle Encyclopedia.