1/3 + 1/4 is not 2/7! You cannot simply add the denominators and numerators of fractions with different denominators. First, you need to find a common denominator so that the pieces are the same size. Use the sliders to make two fractions, choose an operation, and use the bar model to see the whole process from finding a common denominator → calculating → simplifying the result.
Why do we need a common denominator first? 1/3 means "1 piece out of a whole divided into 3 pieces," while 1/4 means "1 piece out of a whole divided into 4 pieces." Because the size of each piece is different, simply adding the numerators and saying the answer is 2/7 is completely wrong.
So we use the least common multiple of the two denominators to make the pieces the same size, and then add or subtract only the numerators. The key is to keep the denominator the same.
The two rows of bars below show the original fractions before finding a common denominator, and the thick bar at the bottom shows the result after finding a common denominator and completing the calculation. Watch how the number of pieces becomes the same.