I gave that a try:

> In each transaction, the spender and receiver each reveal a public key or address to the other person. Afterwards, either person can use the public block chain to track past and future transactions made with that same public key or address, weakening each person's financial privacy.

Do you see the inaccuracy?  To make it correct, we'd have to say something unwieldy like: 

> In each transaction, the spender and receiver each reveal a public key or address to the other person. Afterwards, either person can use the public block chain to track past and future transactions made with that same public key or address (or the public key used to generate that address), weakening each person's financial privacy.