So I have 2 <div> tags in my body, and I want them both to be in one line. However, it automatically makes a line break. Is there a way to fix this?

So I have 2 <div> tags in my body, and I want them both to be in one line. However, it automatically makes a line break. Is there a way to fix this?

.firstDiv {
display: flex;
justify-content: center;
align-items: center;
width: 100px;
height: 150px;
border-radius: 30%;
background-color: grey;
}
.secondDiv {
display: flex;
justify-content: center;
align-items: center;
width: 100px;
height: 150px;
border-radius: 30%;
background-color: grey;
margin-left: 10px;
}
.mainDiv {
display:flex;
flex-direction: row;
}
<div class="mainDiv">
<div class="firstDiv">
First Div
</div>
<div class="secondDiv">
Second Div
</div>
</div>
Use display inline block
.content {
width: 100px;
height: 100px;
display: inline-block;
background-color: black;
}
<div class="content">Content</div>
<div class="content">Content</div>