Here's something fun that might be useful for a fictional universe or something: I'm cataloging seven-part mixtures of the four classical elements. For example, this lil guy is one part Water, two parts Earth, three parts Fire, and one part Air:

We need to be systematic about this, so let's split the elements into "hot" (Fire and Air) and "cold" (Water and Earth). Here are all the mixtures that are either 100% cold (left) or 100% hot (right):

The right column is just the left column with each element swapped with its opposite (Fire/Water and Earth/Air).
If we have some nontrivial combination of cold and hot, like a 3:4 mix, we get something like this:

The left half is 4 parts hot to 3 parts cold, with Air gradually replacing Fire as you move right and Earth replacing Water as you move down. The right half (4 parts cold to 3 parts hot) is a copy of the left half with opposite elements swapped.
So we have a system! There are two other grids like this, for 1:6 and 2:5 mixes of hot and cold. Let's see what a full table of our elemental mixtures looks like:

hey wait a minute

We need to be systematic about this, so let's split the elements into "hot" (Fire and Air) and "cold" (Water and Earth). Here are all the mixtures that are either 100% cold (left) or 100% hot (right):

The right column is just the left column with each element swapped with its opposite (Fire/Water and Earth/Air).
If we have some nontrivial combination of cold and hot, like a 3:4 mix, we get something like this:

The left half is 4 parts hot to 3 parts cold, with Air gradually replacing Fire as you move right and Earth replacing Water as you move down. The right half (4 parts cold to 3 parts hot) is a copy of the left half with opposite elements swapped.
So we have a system! There are two other grids like this, for 1:6 and 2:5 mixes of hot and cold. Let's see what a full table of our elemental mixtures looks like:

hey wait a minute