Your tongue is connected to the floor of your mouth by a vertical piece of tissue called the lingual frenulum (舌系带). In some people the lingual fraenulum is too short, and this condition causes speech impediments. This can be corrected through a specific surgery which clips the tissue, allowing for freer movement of the tongue, and more ease of articulation.
This condition raises the question: Is it the tongue that is responsible for our inability to say tongue twisters like "toy boat" three times fast? Is the lingual frenulum too restrictive to allow for such rapid movement? Or is it a glitch in our brains that accounts for the phenomenon of tongue twisters?
This question has gone unresolved for many years, but some studies point to the brain as the culprit. Evidence to support this came first from a 1982 experiment conducted by researchers Ralph Haber and Lyn Haber. In their experiment, college-age test subjects silently read sentences containing tongue twisters. Sentences that were similar in complexity, but which did not contain tongue twisters, were used as a control.
Interestingly, the Habers found that it took longer for the subjects to silently read sentences featuring tongue-twisters than it did sentences that did not contain them. What's important about the findings is that if our tongues are to blame, then we should have no problem silently reading sentences with tongue twisters. The fact that we don't read tongue-twister sentences as quickly suggests that the brain is the culprit for our troubles with similar-sounding sentences.
The findings indicate that phonology plays a major role not only in the way we say words, but also how we process them while reading. When we read, one of the ways we sort them into comprehensible packages is by the arrangement of the sounds, or phonemes.
When we're presented with phonemes strung together in difficult arrangements -- such as the /oy/ leading to /b/ when repeating "toy boat" -- our tendency to rely in part on phonology may be what trips us up, especially when coupled with the request to articulate the words in a rapid-fire manner.
It's also possible that your tongue simply isn't capable of performing the articulation of "toy boat" quickly. /Oy/ is tricky to say before a /b/. To make the /oy/ sound, the middle your tongue contacts the top of your palate. For the /b/ sound, your tongue must return to its resting place at the floor of your mouth, and your lips must part. This difficult movement -- especially in quick succession -- could also account for this particular tongue twister.
Regardless of whether it's a physical or a cognitive error that is responsible for tongue twisters, don't feel too badly about not being able to say "toy boat" three times fast. You're most likely still a very good speaker. If you're an average person, you make only about one error for every thousand words you say.
（来源：howstuffworks.com 实习生朱一枫 英语点津 Annabel 编辑）