Because your mommy likes to pretend that she doesn't do magic.
3 seconds sound clip from the Practical Magic (1998) movie soundboard.
You can hear this line at 01:05:48.603 in the Blu-ray version of the movie.
Quote context
[...]
- How's about I come by your house tomorrow morning?
- Fine.
- Okay.
- Ten AM?
- Fine.
- Okay. That's a date. All right.
- Jimmy, is that you? Jimmy? Go away.
- Okay. To banish unwanted persons, it says you need blessing seeds.
- All right. Good. Where are they?
- Blessing seeds... What about nigella seeds?
- Good, good. You're good at this.
- Why can't we tell Mommy we're gonna send the policeman away?
- Because your mommy likes to pretend that she doesn't do magic.
- And we have to banish this man for your mommy's own good.
- Blessing seeds. Right, right. What else do we need?
- Antonia, I told you to listen out the door for Mr. Hallet.
- Uh, blessing seeds, blessing seeds, right.
[...]