Federal law requires employers to exempt religious employees from various kinds of generally applicable rules, so long as those exemptions are relatively easy and cheap for the employer. A classic example is an employer no-headgear rule: If an Orthodox Jew, a Sikh, a Muslim or anyone else who belongs to a religion that bars its adherents from going bareheaded asks the employer for an exemption from the rule, the employer must give it, unless there are substantial reasons to insist on the rule (e.g., in the rare cases in which the headgear would interfere with safety). Likewise, if an employer requires employees to work Saturdays, it must exempt Sabbath-observant Jews and Seventh-day Adventists, if such an exemption is relatively cheap (e.g., it can be accommodated through shift swaps that other employees would willingly accept).