You're right - it is normal for kids to be curious. And it's also normal for them to discover the pleasurable sensations of their body. Once they do, many small children get "hooked" on the stress-relieving qualities of self-stimulation. They've learned how to make themselves feel good and feel relaxed. At the same time, however, they know that this activity is somehow wrong: teachers and parents discourage it (at least in public) and other kids don't sit around doing it in front of the T.V. so it can't be totally O.K. And they kids are right when they recognize that this is an activity that both is restricted to the private realm and that somehow needs to be limited. When parents don't step in, kids can actually become addicted to self-stimulation, doing it during every period of "down time." However, parents SHOULD step in for a couple of reasons. First they need to "socialize" their child - teach him how to behave appropriately in public. Parents teach kids which behaviors are socially acceptable and which are not. Some behaviors are deemed "private" like going to the bathroom or being naked. Second, parents need to help their young children regulate their emotions. This means that they need to discourage addictive behaviors and steer the child toward healthier uses of his time and/or healthier ways to relieve stress. For instance, giving a child some crayons and paper or playdough can be helpful for unwinding, releasing stress and using creativity instead of spacing out with mindless self-stimulation. While most kids of this age who play with themselves are perfectly normal and healthy, there are some who have developed the habit due to some form of sexual abuse. If gentle but persistent efforts to re-direct your child's attention fail to "cure" him of this habit, do have him psychologically assessed just to make sure everything is in order.  All the best,  Sarah Chana