Chemicals

 

Source - https://xkcd.com/2648/

See, it's funny because just knowing which atoms - and even how many of each of those atoms - constitute a molecule doesn't mean that you can synthesize that molecule.

There's an analogy here to cooking. If I list the ingredients - even if I include the amounts of each ingredient - for a recipe, that doesn't give you enough information actually to make that final product. Listing the ingredients for a chocolate cake, for example, doesn't tell you in which order the ingredients should be assembled, nor does it tell you how to handle each of those ingredients. Do you whip the butter, fold it in, melt it, freeze it, crumble it into the flour?

That analogy, though, even undersells the complexity of molecules, however, because not all atoms will assemble in the ways we want them to assemble. Some molecules require very specific pathways to produce. 

To read this explanation in far more words, check out explainxkcd.com.