In public key infrastructures a common problem is that both communication partners have to have the same key in order communicate in a secure way (to encrypt and decrypt messages properly). The question is how is it practically possible to share a secret key over an untrusted channel like the internet for example. The solution is the use of asynchronous methods (public and private keys) in order to exchange the secret key in a secure way. In the following video Professor Christopher M. Bishop (Distinguished Scientist at Microsoft Research Cambridge) explains this method in a handy and understandable way to a group of children.
The blue key in the video represents the secret (shared secret). The red key of Andy is his private key and the green key is the private key of Christopher. I guess the public keys are omitted in the video for didactical reasons. But the video demonstrates the key exchange in an easy to understand manner.