Water is by far the most common solvent used in chemistry experiments. When you dissolve a strong acid in water, the acid quickly gives away its proton. The water molecule accepts this proton to become the hydronium ion, written as H3O+.
The hydronium ion is known as the conjugate acid of water. Because all strong acids completely react to form hydronium, they all seem to have the exact same strength. The water essentially levels all of these different acids down to the strength of hydronium.
A similar limiting effect happens when you mix strong bases into water. A strong base will aggressively steal a proton from a nearby water molecule. This action leaves behind a hydroxide ion, which is written as OH-.
The hydroxide ion serves as the conjugate base for all liquid water. Therefore, no base mixed in water can act stronger than the hydroxide ion. Students often forget that this leveling effect changes if you use a totally different solvent.
If you use liquid ammonia instead of water, the strength limits will completely change. The strongest possible acid is always determined by the specific liquid you use for mixing.