Today is Wednesday. What day of the week will it be 100 days from today?

Days of the week repeat every 7 days, so only the remainder matters. $100=14\times 7+2$, so 98 of the days form whole weeks and just 2 days are left over. Counting on 2 days from Wednesday gives Thursday, then Friday. The traps: Thursday uses a remainder of 1 and Saturday a remainder of 3 — both come from a slip in dividing 100 by 7.