Interest
Interest can be your friend in terms of savings and watching your money grow, but more likely your enemy, when it comes to loans, credit card debts and of course the biggest of them all - the mortgage.
In general over the course of a lifetime interest will set you back more than you make on savings (in general) for someone of an average salary.
Interst in terms of the definition is extra money that is paid in order to have someone else's funds: the bank has yours with regard to savings, and you have its with regard to a loan.
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