What makes Acrisios sure he will never have grandson?

AcrisiOS is convinced he will never have a grandson because of a prophecy that foretells he will be killed by his own grandson. To prevent this fate, he locks away his daughter, Danaë, to keep her from having children. Despite his efforts, fate intervenes when Zeus visits Danaë in the form of golden rain, resulting in the birth of her son, Perseus, thus fulfilling the prophecy he desperately tried to avoid.