You can show them how your mouth works - hold your left hand like a letter C (your mouth), and with your right index finger (the tongue) hit the top of the 'C' - this is how we say /l/. Then, for /r/ you show the right index finger just curling upwards, but not touching. Then, you could practise this as a class: the students saying "la la la" quickly (like singing), or 'grrrrrrrrr' (like a dog).
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