I think it’s how they are inside and get stuck
        
                    
             
        
        
        
Pollination occurs when pollen grains from the anther land on a stigma. After pollen grains land on the stigma, a pollen tube grows from the pollen grain, through the style, and into the ovary. Sperm cells inside the pollen grain travel down the pollen tube and into the ovary which contains the ovules.
 
        
             
        
        
        
The organs forming the male duct system as listed from the male gonad to the body exterior are;
Epididymis = Vas deferens = Urethra
Epididymis is a tube located at the back of the testicles that stores and carries sperm. Vas deferens transports sperm from the epididymis to the ejaculatory ducts in anticipation of  ejaculation. The urethra carries urine from the bladder to outside of the body. Additionally, it has function of ejaculating semen when the man reaches orgasm. 
        
             
        
        
        
The answer  is   molecules  <span>nitrogen</span>