Complex sentence: For instance, a water molecule may be small enough to pass through the pores while larger molecules are not, so water is forced into the pores which act as a trap for the penetrating water molecules which are retained within the pores.
Split into simpler sentences: For instance, a water molecule may be small enough to pass through the pores while larger molecules are not. The pores also act as a trap for the penetrating water molecules which are retained within the pores.