A researcher would like to test the relative effectiveness of praise versus candy as a reward for learning a cognitive skill. Nine children were randomly divided into two groups. All were given a long series of problems to solve (the 'learning set'). Each problem involved the same cognitive skill to solve it. After the series of problems in which the children were learning the cognitive skill, they were given a set of 10 more problems to solve that also required that skill. The number of problems they solved correctly in this last ('testing') set was recorded as a measure of how well they had learned the skill. The children in the 'praise' group were given praise after each problem they solved correctly during the learning phase, while the children in the 'candy' group were given a piece of candy after each problem they solved correctly during the learning phase. The data are given below:

Group 1 (Praise): 8, 7, 6, 9, 8

Group 2 (Candy): 6, 7, 5, 4

find the t value for the (M1-M2) of the experiment

t=?