The First Law of Thermodynamics tells us that energy can’t be created or destroyed. It can only change from one type to another. Let’s break it down with a couple of examples:

1. Mechanical Energy: Think about a roller coaster. When the coaster goes up, it’s changing its speed energy (called kinetic energy) into stored energy (called gravitational potential energy).

2. Heat Energy: When you rub your hands together, the movement (mechanical energy) changes into heat energy. This makes your hands warm.

In both of these examples, the total amount of energy stays the same. This idea is called energy conservation.

We can describe this with a simple formula:

$\Delta U = Q - W$

Here, $\Delta U$ means the change in internal energy, $Q$ is the heat added, and $W$ is the work done.