6 year old boys show private parts to each other

Is there anything about 2 6 year old boys showing their private parts to each other?  What is the recommended response of the parents?

[socgirl]

curiosity about the human body is a normal part of child development.  As long as it's "safe play" i.e. "i'll show you mine if you show me yours,etc", then acknowledge this as a part of normal development.  Emphasize that it's ok to be curious about the body.  This is also a good time to stress that it's not ok to touch other people's private part, etc.  It's a phase and it will pass, if it hasn't already.