In the Christian Bible's New Testament, the four gospels tell pretty much the same story, which makes the differences between them that much more intriguing. The Gospel of Mark in particular is famous for being very short — the story cuts out at the end before Jesus ascends to heaven, which seems like a pretty drastic omission.
Instead, Mary Magdelene and the other mourning women simply approach Jesus' tomb to discover the stone has rolled away. A figure in white tells them that Jesus has risen, and they scatter in alarm. End scene. Compare this to the Gospel of Luke, 24: 50-53, which ends, "When he had led them out to the vicinity of Bethany, he lifted up his hands and blessed them. While he was blessing them, he left them and was taken up into heaven. Then they worshiped him and returned to Jerusalem with great joy. And they stayed continually at the temple, praising God."
Depending on which version of the Bible you've read, you may not have noticed this difference — some copies, including the King James Version, have an extra section to fill the gap. There are four different possible endings to the text, which have come down from antiquity in different copies, but the earliest versions of Mark 16 do not include verses 9 through 20. Today it is generally acknowledged that the endings were added later to make up for the abruptness of the text. So why is Mark's story so short?