Did you know lemons contain more sugar than strawberries? :)
A lemon contains 70% sugar and strawberry contain only 40% of sugar. The reason that the lemon tastes sour even having more sugar than the strawberry is that the lemon contains a high amount of citric acid, usually 3% to 6%, that dominated the sweet taste of the lemon and results in the sour taste. The strawberry has less sugar than lemon but there is no acid content present in the strawberry, and therefore it tastes sweet.