Knots

Anya gave me her baby so I could untie the ribbon knotted around its waist, and said:

"Mommy, do you know why I can't untie some knots?  I can untie easy knots, but if I try to untie hard ones, my tummy hurts.  That's why I can't untie this knot.  It's a hard one, and I don't want my tummy to hurt."

Ah.  I see.