Here are some similar questions and how they were answered:

http://answers.yahoo.com/question/index?qid=20071217153921AAMbYY8

http://answers.yahoo.com/question/index?qid=20091119105554AAU9qjC

http://answers.yahoo.com/question/index?qid=20091023031109AAD8K1i

On a less serious note:

Depends on the size and shape of my hands and how soft the gound is.

At that rate doing the same exact thing, your body might be getting use to the exercise and has conditioned to expect 100 push ups every other night. I heard its also called coming to a plateau.

If you want better results you might want to consider changing it up? Make sure the exercise your doing is targeting the muscles you want or need.

@Steve_A Yes, it is called a plateau. But for better results, you want to increase the repetitions slowly over time to continue seeing results. If you continue to do 100 every other night, you will not gain more muscle mass, strength, etc. But you will effectively keep those things at an even level if the amount is not increased. Do not increase the repetitions every time you work out. Increase the amount every 2–3 workouts, other wise you will not have time to succesfully adapt to the increased workload.

When my daughter was very small, she like to sit on my back when I did push-ups, by the time she was 12 (and lost interest) my arms looked like legs.

don’t do them too early in the morning, the muscles just aren’t ready.
For developing arm strength, punching power, strong shoulder strength,
stronger chest muscles…,what can you say? They are great.
But as well to remember, they are only one part of a complete picture….,for balance.

The first thing you need to define IMO is what you mean by “results”. Are you trying to increase muscle size? which muscles, just your chest or overall development? Or are you just trying to go for strength and endurance?.
Doing 100 push-ups is great for core strength and endurance but won’t do much for muscle size, and won’t increase your pressing strength. In order to increase strength and size you need to use progressive resistance. This means that you must periodically increase the amount of weight used to perform the exercise. This is hard to do with push-ups unless you use the method mentioned by Dr. Bill (put someone on your back).