First, because everyone is different, so teachers should open mind to accept the opinions of others and adapt to every situation. Listen to disadvantages and improved. Ready to learn and develop. It makes the teachers are modern and not die with the same old idea. It means teachers must be flexible in themselves.
Second, students at present and in the future will be different from the past because the environments affect the thoughts and actions of the students, such as all students have mobile phones or students addicted to computer games. Therefore, teachers need to be flexible to understand and accept the changes of each student.
Third, due to environmental change, including the students have the change. So teachers should be ideas for teaching a variety of focus on the students to learn by themselves, because learning by themselves will make students real knowledge and can apply in daily life. It means teachers need to be flexible in learning management.
In summary, the most important characteristic of the teachers in the 21st century is that teachers should be flexible, can adapt to environmental changes, ready to accept and learn about students' changes including agreeing to update and improve their teaching regularly. To show that teachers did not die along with the generalizations of old society values.