Transubstantiation
This started when Jesus ate a meal called the Last Supper the night before he died. He said that the food his friends were eating was his body and blood. In the Gospel of Matthew 26:26, it says, "Take this [bread] all of you and eat it. This is my body which will be given up for you," which means that he would die for them. Then, in Matthew 26:27, Jesus said, "Take this [cup] all of you and drink from it. This is the cup of my blood, the blood of the new and everlasting covenant." This means that Jesus's blood would make a promise forever between God and his people. Then he said, "My blood will be shed for you and for all so that sins may be forgiven." This means that Jesus would die for everyone so their sins, or things they do wrong, could be forgiven. Then he said, "Do this in memory of me." This is Jesus commanding people to continue what he did. From that time until now, people have continued what Jesus did and it reminds them of the Last Supper.