Why can't you fill a tank up to 100 with propane?

Why can't you fill a tank up to 100 with propane?
Answer 1Actually you can, but tanks are filled to around 85% to allow for expantion of liquid propane. As the temperature increase liquid propane can expand 5-10%. Example if you would fill a tank to 100% in the morning when its 60 degrees and the temperature during the day increased to 90 degrees the liquid propane would expand to the point that the relief valve would release the liquid that will not fit in the tank. Answer 2Liquified Petroleum Gases [LPG], including propane, expand as their temperature increases.
IF a tank were filled to 100% of capacity, then increases in temperature would result in pressure increases on the tank of hundreds, or possibly thousands, of PSI, and possibly cause a very severe mechanical failure resulting in a massive fire and/or chemical explosion.
This is because liquids have very little compressability, while gasses like air, or LPG in gasseous form, are very highly compressable.
Safety codes require that some empty space remain above the liquid after "filling" of a tank. Then moderate pressure increases due to moderate temperature increases are safely absorbed, or buffered, allowing only moderate pressure increases to the tank.
As pointed out in answer 1, it is theoretically possible of phically and totally fill a tank, but if proper safety valving and filling techniques are used, then there will always be a "headspace" for safety in the tank at all times.