#transition-container {
  width: 100vw;
  height: 100vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  font-family: Arial, sans-serif;
}

button {
  width: 158px;
  height: 55px;
  border: 1px solid black;
  border-radius: 5px;
  background-color: white;
  color: black;
  font-size: small;
  text-align: center;
}

/* SOLUTION */

/* disclaimer: duplicating the `button` element here isn't best practice.
In your solution you probably put it right inside the existing `button`,
which _is_ the best practice. 
We separated it out here to make it extra clear what has changed. */

button {
  transition: font-size 1s ease-out 0.25s;
}

button:hover {
  font-size: x-large;
  cursor: pointer;
}