One way to let your crush see that you like him without opening your mouth is to simply lock eyes with him. Make eye contact for just one or two seconds—just enough time to show you’re paying attention—and then look down at the floor or just avert your gaze. This will show your crush that you like him without being too obvious; if you simply stare at him, that may be coming on too strong.  You can try this trick just one or two times when you’re in the same room together, whether you’re at a party or in class. Just try to be subtle about it; you don’t want him to catch you staring too many times! You can even give him a small flirtatious smile to go along with the look. You don’t need to give your crush an ear-to-ear grin to show him that you’re interested. Simply look at him, give him a small smile, and then look away or start a conversation with him. Your smile shouldn’t be too big or eager, or he may just think you’re being friendly; instead, try to be a little bit mysterious when you smile at him, so you play coy while letting him know you’re interested. If you really want to up your flirtation factor, you can even learn to smile with your eyes. Another way to let your crush know you like him is through some simple open body language. If your body is “open,” that means that you’re standing tall, face your crush, turn your feet and shoulders toward him, and keep your arms out instead of crossed over your chest. This shows your crush that you’re interested in him and that you like being around him. The next time you’re standing next to him or sitting next to him, make sure you’re turning your body in his direction so he sees that you like him. For example, if you’re talking to him and cross your legs, make sure your crossed leg is crossed toward him, not away, so you have more of an open posture. Another way to show your crush that you like him is to simply move a bit closer to him, whether you’re talking to him or just hanging out at the same party. You don’t need to be too forward or start close-talking...

TL;DR in Vietnamese:
Nhìn vào mắt người ấy rồi dời mắt đi nơi khác. Mỉm cười. Xoay người về phía người đó. Đến gần chàng hơn. Đụng chạm. Có vẻ ngoài thật tuyệt.