From the Super Mario Wiki
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Ninja Guys stand in one place, permanently facing one direction. They produce shuriken-like objects from their back and use them to attack Yoshi when he draws near. They only attack Yoshi if he is located in front of them, and throw them at regular intervals. The shurikens travel on a straight line and can damage Yoshi on impact. Yoshi is also damaged if he runs into a Ninja Guy without attacking it. However, the enemies can be defeated by any means of attack, such as jumping, throwing an egg or eating them and turning them into eggs.