I also remember having classes with other professors where all I remember of their lectures was that they were excruciatingly boring. Although demonstrations are of great value in teaching scouting skills and other such things, they are even more value when God demonstrates the supernatural among us in response to our obedience and faith. When you see people get up out of wheel chairs and be instantly healed, that gets your attention. When you see someone's too-short leg grow to match the other one under the power of God, it gets your attention. When these miracles are associated with teaching, the miracles help people remember the message.
Many people teach and preach.
They often preach traditions of men, religious thinking, some truth, some half-truths, and even some outright falsehoods that sound good but are not. Others teach outright lies and false gods. Some of these even have some kind of demonstrations, perhaps of demonic power or natural trickery. To really see signs following you, just preach the gospel. It is that simple.