Skeumorphisim

Hey guys I’m taking the css navigation course and I’m wondering about the following code.

.answer:active {
margin-top: 24px;
margin-bottom: 16px;
box-shadow: 0px 0px;
background-color: #C0D6DF;
color: #ffffff;
}

Why when I remove the margin bottom value does it push all the buttons down when I click it and just one button when it is there? Was wondering why we needed it at all but then I noticed that all the buttons were pushed down when they were clicked.