Tongue twister - Why is it so hard to say?

Try these fast

She sells seashells by the seashore.

The shells she sells are surely seashells.

So if she sells shells on the seashore,

I'm sure she sells seashore shells.

So why is it so damn hard to say "She sells seashells by the seashore" five times fast?

When researchers at the University of Illinois asked college students to read tongue twisters alongside everyday sentences, they found that in addition to being difficult to say, these dastardly phrases take longer to write on paper.

What does this mean? That we trip over tongue twisters because they trigger a cognitive failure, not a physical failure.

And now that your tongue is off the hook, it can get back to failing at foreplay.