Okay. So you know how energy systems work. You get a limited amount of energy or fuel or life, and each time you play a level or go off on a mission, you use up some of that energy. Use it all and you'll need to wait or pay to get it back.
Love it or hate it, but this monetisation model is in pretty much every free to play game these days.
But Pokemon Picross, which just hit the Nintendo 3DS eShop, takes it up a notch. Instead of losing energy for every puzzle you attempt, you lose energy just for tapping tiles. Every tile you tap - and you'll need to tap upwards of 100 to finish an average puzzle - eats at your energy.
Which means you can run out of energy, and see that dreaded "pay some premium currency or wait half an hour" screen in the middle of solving a puzzle. Even Candy Crush doesn't do that.
As usual, this business decision affects the game design. In the main campaign, the game only lets you play in "Free Mode" which is a style of Picross where you aren't told about your mistakes (until you get to the end and realise that you got the puzzle wrong), because this will lead to more tiles being tapped.
Personally I prefer Free Mode, but every other Picross game offers you the choice between Free and Standard Mode.
Now I should point out that like all other Nintendo 3DS free to play games, Pokemon Picross offers you the chance to effectively buy your way out of the energy system. Spend enough Picrites in the game (5000, which would cost £28.78) and you'll be able to turn off the energy system.
But that's a lot of scratch for an eShop game. So if you're not willing to pay that much, keep an eye on your energy gauge in this game - it might run out when you least expect it.