Bilayers are formed by most abundant of the lipids in a membrane. There are two kinds of phospholipids: hydrophobic and hydrophilic. In this course of bilayers I like to teach my kids if bilayers are selectively permeable, and about their polarity to substances.

Hydrophobic is a non-polar part of a molecule and hydrophilic is the polar part of a molecule. Bilayers are semi preamble and they have gaps where the hydrophobic tails are, allowing H20 to flow through. They also generally allow non polar uncharged substances to flow through with the exception of water.

Water is polar but it is amphiprotic, it can act as a base or as an acid. Salt is generally acidic, not a base, it is not amphiprotic. I don't think you guys have gotten to this part yet so I don't know if any of this makes sense. But an easy answer would be that the bilayer is selectively permeable.

This webpage's information is from Professor William Preston's own words on what he believes phospholipids bilayer membrane to be.