The spider and the fly

"'Will you walk into my mortgage ?" said the spider to the fly;
"'Tis the prettiest little mortgage that ever you did spy.
The way into my mortgage is up a variable rate,
And I have many curious things to show when by these rules we skate."
"Oh no, no," said the little fly; "to ask me is in vain,
For who accepts your variable rate can ne'er be out of debt again."