Forced to watch ads or given rewards for watching ads.
"Have to watch an ad every time you finish a puzzle, want to restart a puzzle, or want to add more tubes to make the puzzle easier."
"yes, but you can pay a flat fee to permanently get rid of advertisements (although there are still some available to watch for rewards)"
"Just recently seen an inappropriate game ad about zombies. Also you can have a free hint by watching an ad."
"The game gives you rewards for watching ads. There's literally about 50 adds per day if you want all of them."