Day 15 was all about resources for worldbuilding. The website gives a pretty comprehensive list of some great resources to use if you’re interested in finding more worldbuilding tools, so check it out!
Day 16 deals with the limits of your speculative element. That means establishing rules for magic, or boundaries your creatures can or cannot cross, and setting out the repercussions.
I imagine magic in this world acting like another system within the body–a second circulatory system, for instance, with “magical” veins running alongside the blood-filled ones. Magic is mostly inherited. People with two magical parents are very likely to have magic than someone with only one magical parent, but children of magical parents are not necessarily guaranteed to have magic themselves (example: Gareth). Magic is a recessive gene, and so will sometimes crop up among children with non-magical parents who might have had magical ancestors, but this is rare.
There are different types of magic:
Human: able to affect different parts of the human body, whether healing, helping to sleep, incapacitating, dictating movements, knowing thoughts, etc.
Animal: As above, but in regards to animals.
Natural: able to enhance the powers of plants, help things grow, mold rock, open the seas, cleanse water, etc.
Spiritual: Able to communicate with spirits. Extremely rare.
Within each type, there are different levels of magic. I like to think of it like this: people have “little” veins of magic or “big” veins of magic. Those with little veins of magic might appear to have a talent for something: a natural wizard with a smaller gift might always have the winning pumpkin at the fair, for example, but unless they’re well-trained, they wouldn’t know that they were even doing magic.
People with bigger veins are the problem. Without training and control, magic can come out unbidden–it doesn’t take necessarily take concentration on the wizard’s part. People with more magic to expend tend to have bigger accidents before they can get their power under control.
A person must be in physical contact with the object being magicked in order to work magic. However, after the magic has been done, the effects can last a long time without physical contact.
Education and training can make a wizard use his/her gifts more effectively, but it can not increase their power. It is like an artistic gift; some people are born to paint, and other scan study painting and become better at painting, but they’ll likely never become as good as people who have a natural gift.
When doing magic, wizards slowly but surely deplete the magic-blood in their veins. This can happen at the same rate for people with big and small gifts. People with small gifts are typically incapable of doing things people with bigger gifts can, but smaller workings take less than bigger workings do. However, people with bigger magics are accustomed do doing bigger workings–therefore, two wizards with different levels of gifts going about their average day will usually become exhausted at the same time.
When a wizard depletes their magic, the after-effects can be headaches, exhaustion, vomiting, fainting, or death depending on how far they’ve pushed themselves. Well-trained wizards know their limits and are able to stay within reason on the average day. To recuperate, they are prescribed with sleep, bed rest, and diets high in protein. If pushed to their limits, they typically recover some ability in two to three days, but it can take a week or two before they are at full strength.