This answer says God is able to do only things less than God. If you say there exists a mountain that is "heavier than anybody is able to lift," then what you say is funny: it means nothing, because God is able to lift any mountain. This is because saying a mountain is "too heavy to lift" means this mountain cannot be lifted by anybody. This does not mean that God is too weak to lift very heavy mountains. God cannot lift an "unliftable" mountain because that would not make sense. He also cannot create an "unliftable" mountain because that also would not make sense, if God can lift everything. God could still lift any mountain that is not defined as "cannot be lifted." For example, God can make a mountain as heavy as he wants, but he cannot make a round square.