Edit in JSFiddle

<div class="contain">
  <div class="left"></div>
  <div class="left"></div>
</div>
<hr>
<div class="contain BFC">
  <div class="left"></div>
  <div class="left"></div>
</div>

.contain {
  background-color: #eee;
  border: 4px solid red;
}
.left {
  float: left;
  background-color: #444;
  width: 100px;
  height: 100px;
}
.BFC {
  overflow: hidden;
}