Felix Felicis introduces the third day of new Pottermore information on Potions and Cauldrons.

If I knew I could make Felix Felicis, liquid luck, in my chemistry class, I would’ve definitely tried harder in that class. Unfortunately, chemistry was definitely not where my strength lay, and as Harry Potter author J.K. Rowling mentions in her personal thoughts, I left it behind as soon as I could without regret.
But that doesn’t mean the information that what Rowling talks about isn’t interesting enough to read about. Solve the riddle and unlock Day Three to check out the Potions classroom that Horace Slughorn teaches in. Plus, see what Felix Felicis is really supposed to look like!
His potions lessons are full of interesting things
With students keen to see what each day will bring
With his large moustache and rotund shape
Who teaches this class after Professor Snape?
Don’t bother with putting Horace in your answer. As with the first riddle, you’ll have to write it specifically with the person’s title.
Once you’re in, you’ll be able to collect a couple of items within the wall of potions, as well as find the first subject of new information: Potions.

In J.K. Rowling’s personal thoughts on potions, she compares it with chemistry in muggle school:
“Chemistry was my least favourite subject at school, and I gave it up as soon as I could. Naturally, when I was trying to decide which subject Harry’s arch-enemy, Severus Snape, should teach, it had to be the wizarding equivalent.”
Despite her declaration of hatred for the science course, she admitted to finding Potions as interesting as Snape found it and enjoyed creating them in her books. She goes on further to explain how some of what she was was based on real items with real belief in its properties.
Double-click the image to zoom in and you’ll find the information about Cauldrons in there.

In J.K. Rowling’s personal thoughts on the subject, she considered another purpose for one in her books:
“I did consider making Helga Hufflepuff’s hallow a cauldron, but there was something slightly comical and incongruous about having such a large and heavy Horcrux; I wanted the objects Harry had to find to be smaller and more portable.”
(Source)