Edit in JSFiddle

<div class="wrap">
<div class="father">
  <div class="float">
    我是内容
  </div>
</div>
</div>

<div class="wrap">
  <div class="father">
    <div class="float">
     我是内容，我是内容，我是内容
    </div>
  </div>
</div>

.wrap {
  margin-bottom: 10px;
}
.father { 
  width: 100px; 
  height: 100px;
  background-color: gray;
}
.float { 
  float: left;
  background-color: #ff00ff;
}