img { 
    max-width: 100%; 
}

body {
    margin-left: auto; margin-right: auto;
    width: 90%;
    max-width: 40rem;
}

@media (min-width: 640px) {
  .two-columns-example {
    display: flex;
  }

  .two-columns-example > * {
    width: 50%;
  }
}