Yes, thats the magnet rock. He stays in there all day, they all love it.
The bottem of it is flat and will only move back like .25 of an inch when you take of the viewing part.
What I do, is I have sand in that cage so I pull some sand to the back of it and make it stationary.
You don't realy need to worry about hirting them when taking the cap on or off.
The magnet is REALY strong, if you let the two pieces get together without glass in the middle, you will need a rinch to get them apart.
I only got that one and the next sided down becasue A pat store in my mall was closing and they where selling them for $7 each!
that saved me 50 dallors. 
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Austin-WiteLeo
Leopard Geckos
3.2.2
Brothers
2