A milk can

Salty1

Planter
Disclaimer: not trying to tear your idea apart, but spitballing how it could/might work.

Small milk/large milk come in all varying qualities, so that complicates the potential pathways for function to work, but for argument's sake let's just look at base value for now.

small milk = 38 energy and 17 health
large milk = 50 energy and 22 health

small milk x 4 = 152 energy and 68 health
large milk x 3 = 150 energy and 66 health

So we might argue that a good control point would be requiring you to have four small milk in order to yield three large milk. There is a tiny bit of loss, but that is relatively realistic (there's always a little that clings to the side of containers).

But I imagine the motivation of this suggestion is, at least in part, to reduce the number of stacks of milk and to help keep things organised. So you might restrict this by milk quality too, but that would just yield varying stacks of large milk thereafter. Would it make more sense to have the milk can device yield only one specific quality to enhance that convenience? In that case, things naturally get more complicated.

regular small milk = 38 energy and 17 health
silver small milk = 53 energy and 23 health
gold small milk = 68 energy and 30 health
iridium small milk = 98 energy and 44 health

while:

iridium large milk = 130 energy and 58 energy

If we look at those each individually, then

~3.421 regular small milk = 1 iridium milk (with 1 small being about 29% of capacity)
~2.45 silver small milk = 1 iridium large milk (with 1 small being about 41% of capacity)
~1.91 gold small milk = 1 iridium large milk (with 1 small being about 52% of capacity)
~1.33 iridium small milk = 1 iridium large milk ( with 1 small being about 75% of capacity)

Keeping in mind this follows energy thresholds - health thresholds vary slightly from them but we'll try to round otherwise for the sake of convenience.

We might then, say, have you load all your milks and have them convert into "points" when you activate the machine. Small regular is 3 points, small silver is 4 points, small gold is 5 points, small iridium is 8 points. Each time a 10 point mark is hit when the machine is activated, it spits out an iridium quality large milk. If the sum total is not divisible by 10, then the remainder bottles are left sitting in the device until more small bottles are added and it is activated again.

Would something like that fit the ultimate intention behind this request? I feel like it is at least reasonable in the math, with a slight benefit for the lowest quality while the greatest advantage lies with processing the highest quality of the smalls, but feasibility otherwise isn't something I'd be able to discern. I just feel like any such proposal deserves to be flexed and stretched, to see what we might do in reality rather than just sitting back and saying, "Oh yeah, that might be nice".
 
Honestly, I'd be happy if 2 small milk of any quality becomes 1 large milk regular quality.
I would like if it also worked with goat's milk as well as cow's milk.

Also, as @imnvs said, I would be happy taking a loss on the overall value in order to keep things simple and easy.

Suggestion, this could be a feature added to Marnie's house/shop that you unlock after a certain number of hearts or at a set date (Fall 1, Year 1) in the game. That might allow people to complete CC bundles (if they chose that route) early if they had been focusing on friendships rather than farming.
 
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