Carol in Havelock, Ontario has floors that are so springy that glasses tend to jump off the shelves when she walks by.
Springy floors can be perfectly safe, or they can be an indication of a structural danger. Usually they simply mean that the floor is not rigid enough, or stretches too far between supports. In fact we have things called Span Tables that are designed precisely to avoid springy floors, but some contractors push the limits.
You can reduce or stop the spring by adding support below. If you don't want more posts below, you can reinforce the floor with cross bracing between the joists and with strapping across the joists. Sometimes that is enough.
The Canadian Wood Council in Ottawa publishes the book called the Span Tables. There are some animations about floor problems like this on the CD-ROM included with my book Just Ask Jon Eakes.