Edit in JSFiddle

<p>example</p>

p{
    width:150px;
    height:50px;
    color:#fff;
    background-color:#ccc;
    text-align:center;
    line-height:50px;
    margin:20px ;
    -webkit-transition: all 1s ease-in ;
    -moz-transition: all 1s ease-in ;
    -o-transition: all 1s ease-in ;
    transition: all 1s ease-in ;
}
p:hover{
    width: 350px;
}