body {
  font-family: sans-serif;
  text-align: center;
}

button {
  margin-bottom: 25px;
}

#container {
  display: flex;
  flex-wrap: wrap;
  width: 360px;
  height: 360px;
  margin: 0 auto;
  border: 1px solid black;
}

#container div {
  flex-grow: 1;
  transition: 300ms;
}