Predict the result
Find the intersection of these two number sets.
Which numbers appear in both?

Set up the model
Intersection keeps only elements common to both sets.
Check each element for both memberships.

Work the example
Three, five, and seven appear in both sets.
One and nine occur only in A, while two occurs only in B.
The intersection is the set containing three, five, and seven.
Listing an element once is enough.

Check the result
Now intersect the source letter sets h, a, p, y and s, a, d.
Take eight seconds to think before the answer appears.
[Eight-second silent pause to work; the answer remains hidden.]
Only the letter a appears in both sets.
The intersection is the one-element set containing a.

Interpret the result
Do not confuse both-set membership with the either-set rule for union.
Use the original task and check that your result fits its meaning.