Learning cantrips

Anybody can learn cantrips—including both PCs in non-spellcasting classes, and random non-adventurer NPCs. It's difficult and risky, so many people won't bother, but it's possible to meet an NPC who knows a handful of cantrips.
This requires finding a way to learn the cantrip, though a teacher/scroll/divine message/etc. Learning a cantrip then requires a successful Int/Wis/Cha roll against DC 10, depending on the type of magic (characters already in a spellcasting class add their proficiency bonus). If you don't already know any cantrips, roll with disadvantage.
If your attempt is unsuccessful, you can't try to learn the same cantrip again until you've gone up a level.
Whether successful or not, the attempt also triggers a Wis saving throw against DC 10; losing means you acquire a mental disability, such as a phobia. (See the Limitless Heroics expansion for disabilities and their effects. Essentially, learning a spell without training is a form of mental trauma)
Characters not in a spellcasting class can learn a number of cantrips equal to 1 plus their spellcasting ability modifier.
For spellcasting classes, learning new cantrips ad hoc means:

• If the cantrip is on the spell list for your class, add your proficiency modifier to the roll to learn the cantrip.
• For cantrips in your spell list, you don't need to make the Wis saving throw against mental trauma, but for other cantrips, you do—e.g., a wizard learning a clerical cantrip.
• Every cantrip over your existing limit counts as half a spell, rounded down, against your available spell slots. For instance, if you're naturally allowed 4 cantrips, but you know 6, then 2 of those cantrips are taking up one 1st-level spell slot. If you know 5 cantrips, the fifth cantrip is a freebie.
• Un-learning cantrips is, at the GM's discretion, impossible, so give up those spell slots wisely.

Note: This rule was created mostly to allow for the existence NPCs who know a cantrip or two. There are plenty of opportunities for little dramas, like the cobbler who tried to learn the mending spell, and is now narcoleptic. For PCs, learning extra cantrips may well be too fiddly to bother with.