歪星撞地球 六 3rd rock from the sun Season 6 (2000)

A group of aliens has come to Earth to learn about its population, customs, etc. To avoid detection, they have taken on human form which gives them human emotions, physical needs etc. WITHOUT the understanding of what they mean or the inhibitions normally present in humans. Their leader takes the position of a college professor, their military expert as his sister, their intelligence expert, supposedly oldest of group takes form of his teenage son. The uninhibited reactions turn everyday events into unusual situations.