One of the most frustrating times when growing tomatoes is when the flowers drop off of their stems - it's called blossom drop.
Here you can see where the flowers were and the reason they drop of is because they fail to pollinate or set.
There are a number of reasons why this can happen.
The air is too dry or the air is too humid are two common reasons.
One way that helps avoid this happening is to spray the flowers with a mist of water and tap the plant or pot gently.
This helps to move the pollen in each of the flowers from the stamen into the pistil in order to fertilize the flower.
Tomatoes are self-fertilizing and contain both the male and female parts.
Bumble Bees are excellent for this as their wing vibration and method of honey collection also helps the process.
When the pollen reaches the bottom of the pistil the flower begins to develop a very small pea-like tomato that grows to its variety size.
(Thank you to the owner of this photo).