The way mopping presently works is that your mop does 1 damage to each pair of footprints that it hits when you do a mop 'swipe'. Now, if your mop is wet, it does like 3 or 5 damage or something to the dirty footprints. These footprints come from players walking around on the back of the whale, by the way. Whenever a footpring is left or when it is mopped up, the whale checks the total number of footprints on its back. If it it's less than 20 or something, it is clean. Greater than 20, mildly dirty and the entire whale sprite gets tinted brown and there is a drag applied to any players walking on the whale's back. If there are more than 40 footprints (now that's a dirty whale!), the whale is considered quite dirty and is tinted an even darker shade of brown and even more drag is applied to players trying to waddle through the muck on deck. So what's depicted above is the lonely red player working all by himself to fly the whale into a nearby rain cloud and giving the whale a bath (and a drink, incidentally). Also, when you get the hose attachment for the whale, which can be used to spray down thirsty dogs, attck fire based enemies, put out fires on burning buildings, it can be also used to manually wet the whale deck, instead of needing to locate a rain cloud or to use drop mops.
Mopping has been in the game a long time, but I felt like talking about it today. It serves a few design purposes. (A) I think it would be great to hear about how a group of friends struggled with the decision to upgrade their mops or their guns. (B) general upkeep task that requires time, competes for resources (water), and affects general gameplay (the drag as you walk on a dirty whale is not insiginficant, especially when you're carrying the heavy sack of dog food). (C) provides another motivation for finding rain clouds. Now, where I think it could perhaps use some refinement is in the visual communication department. People don't notice the footprint specks and/or realize you have to mop up the specks specifically, rather than just the whale in general. I think having to target the footprints is important, because forcing the player to move around, get in the way of other players, and also leave more footprints while mopping, makes the activity more nuanced and more interesting as its own little minigame. And also, just mopping the fin over and over doesn't seem like that should count for cleaning the whale (however, it doesn't seeem like 20 footprints left only on the nose should dirty up the whole whale either...oh well).
Just because you're curious, I'll also let you know that it used to be designed so that only when you walked through a footprint speck did you feel the drag. I basically decided against this, because while I want it to feel bad when you're moving slowly because it's a dirty whale, the stop and go of hitting dirty footprints felt more buggy than an intentional mechanic. Maybe this is because it's generally difficult to see the footprints, especially just one or two, especially when they're underneath your player sprite. With a whale-wide state of drag corresponding to a state of dirtiness as visually communicated by whale color tinting, at least you know what's happening, if you know how mopping and leaving footprints functions in general. Anyway, people think the dirtiness of the whale is indicating damage though. So I don't know. Maybe I'll just adds a bunch of blood splurting out all over the place for when the whale takes damage.