These are tongue twisters I heard decades ago (pre-1980).  They are in their original form, except for the parts in brackets, which I added, and the parts in braces, which I heard more recently.

A flea and a fly in a flue didn’t know what to do.  Said the fly, “Let us flee.”  Said the flea, “Let us fly.”  So they flew through a flaw in the flue.

[Round, rollicking, rolling robbers rarely rob] rubber baby buggy bumpers.

The sixth sheikh’s sixth sheep’s sick.

How much wood would a woodchuck chuck if a woodchuck could chuck wood?  [A woodchuck would chuck as much wood as a woodchuck could chuck if a woodchuck could chuck wood.]

Peter Piper picked a peck of pickled peppers.  {A peck of pickled peppers Peter Piper picked.}  [If Peter Piper picked a peck of pickled peppers, where’s the peck of pickled peppers Peter Piper picked?]

Theophilus Thistle, the unsuccessful thistle-sifter, while sifting a sieveful of unsifted thistles, thrust 3,000 thistles through the thick of his thumb.  Seest that thou, while sifting a sieveful of unsifted thistles, thrust not 3,000 thistles through the thick of thy thumb.  Success to thee, Theophilus Thistle, the unsuccessful thistle-sifter.  (Much of this was my creation, but it is based on a tongue-twister I heard long ago.  I do not remember which parts are the original.)

Round and round the rugged rock the ragged rascal ran.

Toy boat.  Toy boat.  Toy boat.  Toy boat.  Toy boat.  Toy boat.  Toy boat.  Toy boat.  Toy boat.  Toy boat.