Stephen , can you work overtime tonight ?   What's wrong ? We have been working overtime recently .   The holiday is coming so we need to finish our task .   Must we finish our task before the holiday ?   Yes . The contract must be kept .   OK . I know . But Bob wanted to change shifts with me tonight .   Did he say why ?   He said he would celebrate his girlfriend's birthday .   Don't worry . I will tell him about that .   It is convenient for me to work overtime tonight .   Thank you for that . And I will double your pay . 