Referencing the reference? Suppose I have a paper with the following text:


  Roses are red, violets are blue, sugar is sweet, and so are you [3],[4].


As one can see, the author got the information from two other references.
If I want to add this information in my thesis, do I also have to reference [3] and [4] or can I just reference this particular paper, I got the information from?
If you need a reference that supports the factual information that roses are red etc., then you have to look up the original sources and cite those. If, however, you refer to a synthesis of the information that the author of your sample text has derived from the original sources (for example "flowers have different colors"), then you must cite the sample text.

Having said that, it is sometimes advisable to look up the references also in the latter case. First, because the information contained in them might be interpreted in different ways, and second, because you might learn something useful from them. 

Only when you absolutely cannot access the original source, you may resort to a secondary citation, such as [3] as cited in [1].