"Baseball happens to be a game of cumulative tension but football, basketball and hockey are played with hand grenades and machine guns"

John Leonard's quote is a humorous way of comparing the strength of various sports. He is recommending that baseball is a game of stress that develops over time, while other sports such as football, basketball, and hockey are more extreme and explosive. He implies that these sports are played with a greater sense of seriousness and intensity, as if they were being had fun with hand grenades and gatling gun. This comparison is suggested to highlight the difference in the method these sports are played and the level of strength they require. Leonard's quote is a funny way of highlighting the differences between sports and the various levels of strength they require.