Because of the unusual shape of their legs, kangaroos and emus struggle to walk backwards.
A hippopotamus may seem huge but it can still run faster than a man.
Even if an analog clock is broken, at least it shows the correct time twice a day.
Sneezing with your eyes open is impossible.
The trickiest tongue twister in the English language is apparently "Sixth sick sheik’s sixth sheep’s sick". Give it a try and see for yourself.