"The Mistake Test" joke

See if you can do this. Read each line aloud without making any mistakes. If you make a mistake you MUST start over or it won't work.
This is this cat
This is is cat
This is how cat
This is to cat
This is keep cat
This is a cat
This is moron cat
This is busy cat
This is for cat
This is forty cat
This is seconds cat
Now go back and read the THIRD word in each line from the top...