* {
    margin: 0;
    padding: 0;
}

.container1 {
    background-color: gray;
    display: flex;
    width: 900px;
    justify-content: space-between;

}


[class*="child"] {
    background-color: red;
    height: 200px;
    width: 200px;
    border: 5px solid black;
}


.child1:hover {
    background-color: yellow;
    transition: 1s;
}

.child2:hover {
    width: 400px;
    transition: .5s;
}

.child3:hover {
    border-radius: 50%;
    transition: .7s;
}

.child4:hover {
    transition: background-color .5s, border-radius .5s .5s, border .5s 1s;
    background-color: blue;
    border-radius: 50%;
    border: 10px green solid;
}