Tongue Twisters

Today, we presented at our home school. It was a success for the most part. This time, we spoke to 9th and 10th graders about English tongue twisters. We figured that they would be too old to care much for nursery games, so we had to think very quickly again on what to talk about. Luckily, the students at our school understand English fairly well, although they are shy to speak it. Our pupils were really active participants, so it made our job easier. We asked them to repeat after us, translate and then try to quickly pronounce each word in each tongue twister

Try this one out for size:
If you understand, say "understand".
If you don't understand, say "don't understand".
But if you understand and say "don't understand".
How do I understand that you understand? Understand!

Translated into Russian (disregarding grammar):
Если ты понимаешь, скажем, "понимаю".
Если ты не понимаешь, говорят: "Не понимаю".
Но если ты понимаешь, и сказать: "не понимаю".
Как я могу понимаю, что вы понимаете? Поймите!