I want to know why some Data like the ChemicalData once downloaded, the first time you use them, you can work the next time even offline using this Data, I think Mathematica is storing the downloaded Data in a persistent manner, but when I worked with the AircraftData or AirplaneData and many others, it wasn't the case. On the contrary, Mathematica was obliged to download the same Data I worked the last time with again and again, it was a little bit annoying, I can't understand why Mathematica is not storing the Data in this cases? The same issue concerning the Data when I use the Entity functions, every time Mathematica is asking me to download the same Data again and again? ..Why?