There is no such thing as perfect crime, an undetectable crime where the killer gets away free. This is more fiction than reality, as most criminals are given away by even the simplest clues.

There has been an increasing complexity in the way murderers kill their victims. One of the first things a criminal tries to do is to make his/her victim look like s/he had an accidental death.

But forensics do not look just for the cause of death, but also for other minor injuries. Struggle bruising or scratch marks that indicate dragging may point out a murder. Hypostasis is another important clue: after the death, the blood pressure falls and due to the gravity, the blood amasses in the body area that is the closest to the ground.

The red staining clearly shows the body's position at death. A body lying on its right-hand side and with hypostasis on its left-hand side has been clearly moved after the death occurred. "It gives us a very important clue. If you're dumping a body, of course you don't think where's the hypostasis, you just dump the body and run", said pathologist Dr Richard Shepherd.

If a criminal gets rid of the victim's dead body, many clues may be erased.

The "acid bath murderer" John Haigh, who committed his crimes in the '40s, introduced the bodies of six victims in sulphuric acid. But the acid can't dissolve fats or plastic, so a pair of dentures and three human gallstones covered in fat betrayed him.

Sometimes the forensics themselves destroyed the probes. "It was developed for situations like the mass graves in Bosnia Herzegovina, where you find mostly skeletonized remains, but a bit of soft tissue adhering. You need to clean these bones up and to preserve the DNA for identification purposes. The old systems included boiling at high temperatures which was extremely destructive to the DNA", said DNA expert Eleanor Graham at the University of Leicester.

Now the remains are simply heated and gently washed with enzymes that clean off the bones of flesh.

Gunshots, knives or blunt instruments always leave signs on the bones. Still, there is a weapon that does not leave any visible evidence and is preferred by professionals: poison.

The killer can be far gone till his/her victim realizes what's going on. The most famous such case can be considered Alexander Litvinenko's recent poisoning with polonium -210 in 2006. However, the uniqueness of the murder weapon may also give away the criminals, as the alpha radiation leaked by polonium-210 can leave a radioactive trail.

Today's investigators can detect anything from tiny fibers of clothing and shoeprints on a carpet to DNA.

In fact, the way a criminal sheds DNA can help him or not. "Some people are more likely to deposit their DNA than other people. One person could pick up a bottle of water, put it down, you get a full profile. Another one could drink from it and you wouldn't pick up very much. No one knows exactly why this is but there does seem to be a very big difference", said Graham.

That's exactly why, in the end, the "perfect" murder will always be caught: there's too much chance taken in the perfect crime.