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How to know if your rat's teeth are too long
Restrain your rat.
Your rat will not want to hold still while you look at his teeth, so it's important to make sure he is properly restrained. Ideally, you should have a second person hold the rat while you examine his teeth.
When the rat is restrained, you should be able to gently lift his upper lip with your finger to observe his incisors. The simplest way to restrain a rat is by holding him firmly around the torso with two hands.