When he sits on a chair with his eyes open

Winnie can see

Well... she can see his eyes

But when Wilbur closes his eyes and goes to sleep, Winnie can't see him

... and she sits on him

When Wilbur sits on the carpet with his eyes open

Winnie can see him

Well... she can see his eyes

But when Wilbur closes his eyes and goes to sleep

Winnie can't see him

and she trips over him.