This is why one must know his onions.

Actually,  I only just heard this phrase for the first time recently. It’s good advice if you have cats.

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Transcript

Panel 1

Toullie: Sergent, did you know that if you eat an onion, you’re pee smells like an onion?

Panel 2

Sergeant: Toullie! Onions are poisonous to cats!

Panel 3

Toullie: 911? My sister ate an onion!