It states (68:35-36) תנו עוז לאלקים... הוא נותן עוז לעם ותעצומות, "Give strength to Hashem... It is He who grants might and power to the nation..." One of the tzaddikim of our generation asked that it seems like a contradiction. First, we say that we give strength to Hashem, keviyachol, and then it states that Hashem gives strength to His nation!
He answers that people often excuse themselves with the excuse, "I don't have strength. I can't." We tell them, תנו עוז לאלקים, do what you can, and then Hashem will give you strength. Hashem will help you.
We will explain this idea with a mashal: A one-hundred-story skyscraper was constructed in a particular city. An announcement was made that whoever climbs to the top of this enormous building will receive a great reward.
Many people tried to climb to the top, envisioning their reward. But after climbing for a while, they got tired and walked back down. The stronger people climbed ten flights but calculated that since this was just a tenth of the number of flights they had to climb, they, too, turned back down. Only one person kept going up, and with bated breath, he climbed higher and higher. Suddenly, he got to the 100th floor. He stood at the top floor and waved down to the people below. When he came down, they asked him how he climbed up the 100 flights so quickly. He replied that on the eleventh floor, there was an elevator.
Similarly, Hashem has prepared an elevator to bring us up to high levels, but a person must do whatever he can. He must not get discouraged from climbing up. He does what he can, and Hashem will help him the rest of the way.
Reb Avraham Genichovsky zt'l would tell the following story (which he was particularly fond of): The gaon, Reb Yosef Lis zt'l saw a young child standing at the curb of a very busy street where cars were whizzing by. Someone came by and asked the child if he needed help crossing the street. The child stood up straight, as if to appear older than his age and said, "I don't need help. I can cross the street by myself."
A second and a third person came by and offered to help the boy cross the road, but each time, the boy replied that he could manage on his own. Reb Yosef Lis was concerned about the child's safety, so he told the boy, "I am old, and I need help crossing the street. Can you help me?" And the boy was happy to help him. In this clever manner, he helped the boy cross the road.
Hashem also tells the person, "I need help. Please give me strength. Perform mitzvos, תנו עוז לאלקים, give strength to Hashem. Hashem doesn't need our help, but He says He does so that He can help us.
We can say that this is hinted to in the Chazal, "רצה הקב"ה לזכות את ישראל לפיכך הרבה להם תורה ומצוות." The word הרבה can mean that Hashem makes people great. He gives them a feeling of importance. They feel that they are helping Hashem, and with this technique, they perform mitzvos to their own benefit and help.