The challenge provided you with the following list of elements:

Mercury, Lead, Thulium, Polonium, Polonium, Actinium, Americium, Bismuth, Tantalum, Iridium, Mercury, Osmium, Thulium, Americium, Lead, Tantalum, Lutetium, Hafnium, Polonium.

Two things were required to solve this challenge, a periodic table and an ASCII table. ASCII is an encoding for storing text as numbers.

By using the periodic table, you can find the number which corresponds to each of the provided elements. This number can then be looked up in the ASCII table to find out what character this corresponds to. Doing this for every element in the list will let you find the solution which was: PRETTY_SIMPLE_RIGHT.