Tongue Twisters

Tongue Twisters hold a duel purpose. It helps with student's English pronunciation and it provides amusement for me watching students flounder. I use tongue twisters in my lessons mainly for the latter purpose but pretend otherwise.
I first understood the value of using tongue twisters with my Tuesday evening class which consists of two bright High School girls called Yoko and Yunna.
Whenever I teach them, the exercises I set are always finished with relative ease and it gives me the impression they are coasting through my lessons. With this in mind, I needed an exercise that would challenge them. Tongue twisters are great because even if you can't do them, it’s funny for others to watch you fail.
I began with the familiar: She sells sea shells by the sea shore.
Translated by Yoko as: "Sells she sea sea shore she sells.....what?!"
I urged her to relax after the 20th time of her trying to crack this sentence without success. Yunna didn't help matters by laughing hysterically at Yoko's attempts. I turned the tables on Yunna.
"Yunna, please say this sentence," I said, as I wrote the new tongue twister.
Freshly fried fresh fish.
Yunna studied the sentence with care as if it were an important code-breaker.
"Frebbly fwied fred fish," she said.
"Nice try," I consoled.
Yoko was howling with joy.
"Yoko! Say this," I said.
A big black bug bit a big black Bear, made the big black bear bleed blood.
She breathed in a huge amount of oxygen and exhaled with fear, as if I had just asked her to bungee jump from the top of Mount Fuji.
"Big bear bug blood bit blood bear bug bled blood," she said.
"Nope! Yuna, try this."
Friendly frank flips fine flapjacks
"Fwendly fwank fwips fwine fwapjacks."
"Ha ha ha ha. No, no, no. You're all wrong," I laughed with unnecessary gloating malice.
"OK. You do it then," Yoko challenged.
"Me?" I said.
"Yeah!" Yoko and Yunna both said, threateningly.
"Sure," I said, whilst my face twitched with nerves.
They looked at my Tongue Twisters sheet and picked out a horrendously difficult one.
Fred fed Ted bread, and Ted fed Fred bread.
I cleared my throat, assumed a dramatic pose as if I was about to begin a 100 meter sprint, and begun.
"Fred fed bread Ted and Ted Fred bread fed ....class dismissed!"