body {
  margin: 0;
  font-family: Arial, sans-serif;
}

.contenedor {
  display: flex;
  flex-wrap: wrap;
  padding: 20px;
}

.caja {
  flex: 1 1 300px;
  /* background: #d2691e; */
  /* color: white; */
  padding: 20px;
  margin: 10px;
  text-align: center;
  border: 1px solid blue; 
}