This was developed as an age test by an R&D department at Harvard University. Take your time and see if you can read each line aloud without a mistake. Supposedly, the average person over 40 years of age can't do it!

This is this cat
This is is cat
This is how cat
This is to cat
This is keep cat
This is an cat
This is old cat
This is fart cat
This is busy cat
This is for cat
This is forty cat
This is seconds cat

Now go back and read aloud the third word in each line from the top down and I betcha you can't resist passing it on...