need hint

[barabanus]

Hello, everybody!

Could you please give me a hint.
I found the trapdoor and got into the laboratory.
I left the laboratory, but I can't close the trapdoor.
Is there a way to close it?

I clicked the trapdoor ("just walk there"), I clicked the bookshelf (books cycle)
I'm sure it should be quite easy to do!

[adeyke]

Click on the same "books" you used to open it.

[barabanus]

Thank you very much!