Here's the breakdown:

* Peninsula: A peninsula is a piece of land that is surrounded by water on three sides, with a narrow connection to a larger landmass.

* Mainland: Mainland refers to the main body of a country or continent, usually the largest landmass in contrast to islands or other smaller landmasses.

The Connection:

A mainland can be a peninsula, BUT not all mainlands are peninsulas. Here's why:

* Example of a peninsula mainland: Florida, USA is a peninsula because it's surrounded by water on three sides and connected to the rest of the US by a narrow strip of land. It's also the main body of the state, so it's the mainland.

* Example of a mainland that's not a peninsula: The African continent is a large mainland. It is not surrounded by water on three sides, so it's not a peninsula.

In conclusion: While a peninsula can be a mainland, the terms are not interchangeable. A mainland doesn't always have to be a peninsula.