Exodus 20:7 "You shall not take the name of the L-rd, your G‑d in vain"

This Negative Mitzvah teaches not to make a "Shevuat Shav", which means a "vain promise."

A Shevuat Shav can be one of three different types of promises:

1) To swear that something is not what it really is.

For instance: When Sarah got her new bicycle she told her friends that it cost $10,000. If Sarah would have sworn that her bicycle costs $10,000, when it did not - this would have been a Shevuat Shav.

2) To swear that something exists, when such a thing is impossible.

For instance: After Danny went to the zoo with his parents, he told his friends that in the zoo he saw an elephant with wings and the elephant flew through the air. If Danny would have sworn that he had seen such an elephant, which is impossible, he would have made a Shevuat Shav.

3) To swear about something which is plainly obvious.

For instance: To swear that a cow is a cow.

4) To swear that you will not keep one of the Mitzvot in the Torah.

For instance: to swear that you will not say Shema today is considered a Shevuat Shav.

All these promises are part of the prohibition of Shevuat Shav, and neither they, nor anything like them, should be made - even if they are made in a joking manner!