Tigris can open one of our doors. He has to turn the lever down (and it is a door where the lever stays down). So he turns and looks whether he can force it open by paw. If it does not work, he turns again, tries to force the door open.....until it works. I think this repeated thing is a very intelligent behavior.
I knew another one Minouche, daughter of Grisette who just jumped on the lever until the door was open.