A more thorough explanation:

Definition: Mutuum is a type of transaction where goods are delivered, but instead of being returned, they are replaced by other goods of the same kind. This is considered a sale or exchange, not a bailment, because the specific goods are not returned. In Roman law, mutuum was a real contract where money or fungible goods were lent to a borrower, who was strictly liable to return an equivalent amount. Any interest had to be agreed upon beforehand.

• When John borrowed a bag of sugar from his neighbor, he promised to return the same amount of sugar. This is an example of mutuum.
• In ancient Rome, a moneylender would lend coins to a borrower, who would have to return the same amount of coins. This is also an example of mutuum.

These examples illustrate mutuum because in both cases, the borrower is not returning the exact same goods that were borrowed. Instead, they are returning goods of the same kind or value. In the Roman law example, the borrower is strictly liable to return an equivalent amount of money or fungible goods, which means they cannot return something of lesser value.