Opposite Day is an unofficial holiday when every action is modified so that meaning is negated. It is usually observed among children, and rarely lasts an entire day. Once Opposite Day is declared, statements mean the opposite of what they usually mean. Opposite Day can also be declared retroactively to indicate that what was just asserted had the opposite meaning of what was originally intended (similar to the practice of crossed fingers to automatically nullify promises). Paradoxically, the declaration that it is opposite day is negated as well, implying that it is not opposite day. Play has been compared to a children's "philosophy course".