According to the lecturer, what did people used to throw into wells?

The belief that four-leaf clovers are good luck comes from the Druids, ancient residents of the British Isles. Several times a year, they gathered in oak groves to settle legal disputes and offer sacrifices. Then they ended their rituals by hunting for four-leaf clovers. Why? They believed a four-leaf clover enabled its owner to see evil spirits and witches, and therefore avoid them. Ancient people believed spirits living in springs and fountains demanded a tribute--usually flesh. Young Mayan girls, for example, were sometimes tossed into the Well of Sacrifice (where they would "marry" the spirits). Today, we just throw the spirits a penny or two for luck. In the Middle Ages, churchmen insisted that knocking on wood was part of the tradition of prayer, since Christ was crucified on a wooden cross. They were right hut the tradition started several thousands of years earlier, with a different deity. Both Native Americans and ancient Greeks developed the belief (independently) that oak trees were the domains of an important god. By knocking on an oak, they were communicating with him and asking for his forgiveness. The Greeks passed their tradition on to the Romans, and it became part of European lore. The oak’s "power" was eventually transferred to all wood.