Your helmet must stay securely in place if it is to be effective. Make sure that you can adjust the straps so there is no slack. If you helmet is properly fitted you should not be able to move it back and firth, nor should it be possible for it to come forward over the eyes. Riding can be strenuous. It is important to have a good flow of air between the helmet and your head.
Helmets can vary in weight.
The weight of the helmet should bot put too much pressure on the neck and the spine. This is particularly important for young children.