How can Muslim women become slaves?

How can Muslim women become slaves? I heard that it is allowed by Islam to have Muslim women as slaves. I feel this is not right at all, but I want to know for sure. How could that happen? ___ To make it more clear, I'm not asking about non-Muslim slaves that become Muslims after being made slaves, or being born to slaves inheriting the slave status. I am asking about free believing Muslim women becoming enslaved. --- The options from the software are either to delete this question or to edit it. But the software does not allow me to delete it, and editing it in a substantial way would make fool of all the answers and comments already given to the *original, unchanged* question.