Love.

Mathilda: Leon, I think I'm kinda falling in love with you. 

Leon chokes on his glass of milk

Mathilda: It's the first time for me, you know? 

Leon: How do you know it's love if you've never been in love before? 

Mathilda: 'Cause I feel it. 

Leon: Where? 

Mathilda: In my stomach. It's all warm. 
I always had a knot there and now... it's gone. 

Leon: Mathilda, I'm glad you don't have a stomach ache any more. 
I don't think it means anything.