WHy do churches ring bells?

The tradition of ringing bells at Roman Catholic Masses goes back to a time with the Mass was said in Latin. When Masses were in Latin, most people (who couldn't understand the Mass) spent most of the service in quiet, individual prayer. The ringing of the bells signaled that the bread or the wine was about to be blessed and 'turn into the Body or Blood of Jesus' - the central part of the Mass, which required everyone's attention.

Today, the Mass in said in English, but that doesn't always mean everyone is paying attention. They ring the bells to remind the congregation that 'transubstantiation' is about to occur.