A portable electric fan with some bright red streamers/ribbons/tissue paper tied to the guard.
When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them. Acts 2:1-4 (NIV)
On a hot summer day, an electric fan can really help to keep us cool. How does a fan help to keep us cool? That's right, it keeps us cool by blowing air. I think I will turn the fan on now. Do you see any air coming out of the fan? Well, if we can't see the air, how do you know the fan is working? There are several ways we can tell that this fan is working.