"At birth and often in childhood, the foreskin is usually still fused with the glans. With age and usually by the beginning of puberty, gradually they separate themselves and therefore the foreskin can be retracted. In adulthood, the male foreskin is highly variable length. In most cases, the glans is completely covered when the penis is flaccid, while, when the penis becomes erect, the foreskin slips back, leaving the glans exposed. However, as demonstrated by various medical studies, there are several alternative possibilities; as result of the increase in penis size during puberty, the foreskin can not be long enough to cover the whole glans: in this case, it will always be partially (or fully) uncovered. Contrarily, you may have an excessively long foreskin and not very elastic so that it does not slide back by itself with erection".