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The fluid falls under the influence of gravity and is often guided by control of air pressure . Paraphimosis may occur when the foreskin has been pulled back behind the head of the penis for an extended period and is often caused by well-meaning health professionals secondary to penile examination or urethral instrumentation . Hearing impairment associated with noise exposure can occur at any age , including early infancy , and is often characterized by difficulty in understanding speech and the potentially troublesome symptom , tinnitus ( i.e ., ringing in the ears ) . Glacial ice , for instance , forms through the sintering of snow under pressure and is often characterized by equiaxed , randomly oriented grains near the upper part . Many weedy species are widespread , prolific , fast-growing annuals capable of establishing in disturbed habitats and are often favored by disturbances .