1.smudge

There was a dark smudge on his forehead.

2.ignite

The bombs ignited a fire which destroyed some 60 houses.

3.follow suit

He went to bed and I followed suit.

4.scum

She never would have even spoken to scum like him when Mom was alive.

summary:

They talked about who would be the heir of Slytherin. They thought it must be Malfoy, but they had to convince it. Hermione got an idea, going into the Slytherin's common room and asking Malfoy. But they had to pretend themselves the students of Slytherin. Hermione knew one potion could do it but she didn't know the recipe. So they had to got a book, which had recorded it, from the library.