abstract
barrier
basic
brain teaser
brain twister
brainteaser
braintwister
complex
complexity
convolution
decipher
difficult
difficulty
dilemma
disarray
easy
flat
icon
labyrinth
maze
mystery
mystification
obstacle
perplexed
perplexity
print
printable
problem
problem solving
puzzle
puzzlement
puzzler
rebus
regular
riddle
riddler
simple
solution
solvable
solve
solving
tangle
unriddle
unsolved
way