Answer: Albert Einstein's hardest riddle
The best way to tackle the Albert Einstein riddle is systematically. To start with lay out what is known about each of the agents in a grid. In this case the agents are the men who are mentioned in the question. Each man has a number of attributes. These are: nationality; colour; drink; whether or not he smokes a cigar; and whether he has a pet. So, the grid will look like this:
Now in the puzzle it steps up a gear. Up to this point we've got away with adding single facts, but that strategy now leads to a dead end - the information that's left is too sparse. We need to start grouping assertions together to make progress.
Problem is, we could put this into two places - the 2nd column or the 5th - as both fit. We need to look for a clue which rules one out. Luckily, we have this in clue 2. This tells us that the Swede has dogs. So it can't be column two, because that already has horses for the pet.
Now as usually happens, it's all downhill to the end. Clue 15 puts the Blends smoker next to the water drinker. That can only go in column 2, with water going in column 1. That also gives us tea, for column 2. We know the tea drinker is Danish, so in it goes as well.