ASK SCS (June 2017): Any and All Skincare Science Questions!

Do you have a burning question about skincare science that you don't think needs its own thread? Ask here!

Don't be worried if you think your question is dumb - if you're wondering, someone else out there is probably wondering too!

Note:

• Anyone who has an answer can answer, not just the mods! Don't be afraid to take a stab at the answer, a lot of skincare science questions don't have a definitive answer.

• Questions can still have their own threads.

• Routine-related questions are fine, as long as there's a science-related aspect.