A molehill is a small pile of dirt that a mole pushes up from underground when it digs its tunnels. Moles are nearly blind mammals that live beneath the surface, and as they burrow through the soil hunting for worms and insects, they push the extra dirt upward, creating these distinctive little mounds in yards and fields. Each molehill is only a few inches high, though they can be annoying to people who want perfectly smooth lawns.

The word appears in the famous expression “making a mountain out of a molehill,” which means treating a small problem as if it were a huge crisis. If your friend forgets to save you a seat at lunch one time and you decide they must hate you and never want to see you again, you're making a mountain out of a molehill. The phrase captures how easy it is to blow minor setbacks out of proportion, turning something barely noticeable (a molehill) into something massive and dramatic (a mountain).