Logic riddles and brain teasers for kids, each with its assumptions stated, two hints and a worked explanation.
One at a time
Which months have at least 28 days?
Good to know: We use the calendar most of the world uses, where every month has 28 to 31 days. "At least 28" means 28 or more.
Hint 1
Read the question again carefully: it says "at least," not "exactly."
Hint 2 (more help)
Even the shortest month of the year has 28 days.
Reveal the answer
All twelve months.
February is the shortest month, with 28 days (29 in a leap year). Every other month has 30 or 31 days. So every month has at least 28 days: all twelve of them.
Medium riddles
Which months have at least 28 days?
Good to know: We use the calendar most of the world uses, where every month has 28 to 31 days. "At least 28" means 28 or more.
Hint 1
Read the question again carefully: it says "at least," not "exactly."
Hint 2 (more help)
Even the shortest month of the year has 28 days.
Reveal the answer
All twelve months.
February is the shortest month, with 28 days (29 in a leap year). Every other month has 30 or 31 days. So every month has at least 28 days: all twelve of them.
Tricky riddles
Two fathers and two sons share three sandwiches, one sandwich each. How can there be only three people?
Good to know: One person can be a father and a son at the same time, and the three people are from one family.
Hint 1
Can one person be counted twice: once as a father and once as a son?
Hint 2 (more help)
The same man can be a father to his child and a son to his own father.
Reveal the answer
They are a grandfather, his son, and his grandson.
The grandfather is a father. His son is a father too, because he has a son of his own, and he is also a son. The grandson is a son. That makes two fathers and two sons, but only three people, so three sandwiches are enough.
In a race, you pass the runner who is in second place. What place are you in now?
Good to know: You pass only that one runner, and nobody passes you at the same time.
Hint 1
Picture the race: where must you be just before you pass that runner?
Hint 2 (more help)
When you pass someone, you take their spot, and they drop back behind you.
Reveal the answer
Second place.
To pass the runner in second place, you must be right behind them, in third place. When you pass, you take their spot: now you are second, and they are third. The runner in first place is still ahead of you, so the quick answer "first" is not right.
Five apples are on the table. You take two. How many apples did you take?
Good to know: The question asks how many apples you took, not how many are left on the table.
Hint 1
Read the last question very carefully.
Hint 2 (more help)
The question asks about the apples in your hands, not the ones on the table.
Reveal the answer
Two.
Many people answer three, because three apples are left on the table. But the question asks how many apples you took, and you took two.
A ball is in one of three boxes: red, blue, or green. It is not in red or green. Where is it?
Good to know: There is exactly one ball, and it is certainly in one of the three boxes.
Hint 1
Cross out each box the ball cannot be in.
Hint 2 (more help)
Two boxes are crossed out. Which one is left?
Reveal the answer
The blue box.
The ball must be in one of the three boxes. It is not in the red box, and it is not in the green box, so the only box left is the blue one.
In a pretend game, every red block is a cube, and no cube can roll. Can any red block roll in that game?
Good to know: Inside the pretend game, both rules are always true, even if real blocks could tumble. We use only the rules, not real life.
Hint 1
Use only the two rules of the game, even if real blocks act differently.
Hint 2 (more help)
Is a red block a cube? Then what does the second rule say about it?
Reveal the answer
No.
Every red block is a cube, and no cube can roll. So every red block is a cube that cannot roll. In real life you might tip a cube over, but inside this pretend game the rules are always true.
Ali is ahead of Bo in a three-person line. Bo is ahead of Cam. Who is in the middle?
Good to know: The three people stand in one straight line, and "ahead" means closer to the front.
Hint 1
Draw three circles in a row and write a name in each one.
Hint 2 (more help)
Ali is ahead of Bo, and Bo is ahead of Cam. Who is at the very front?
Reveal the answer
Bo.
Ali is ahead of Bo, and Bo is ahead of Cam. So from the front, the line goes Ali, Bo, Cam, and Bo is in the middle.
Challenge riddles
Two US coins total 30 cents. One of them is not a nickel. Which coins are they?
Good to know: Only real US coins count. "One of them is not a nickel" is true of the quarter; it does not mean the other coin cannot be a nickel.
Hint 1
List the coins that could add up to exactly 30 cents with just two of them.
Hint 2 (more help)
The riddle says one coin is not a nickel. It does not say that neither coin is a nickel.
Reveal the answer
A quarter and a nickel. The quarter is the one that is not a nickel.
The only two US coins that make 30 cents are a quarter (25 cents) and a nickel (5 cents). The riddle says one of them is not a nickel, and that is true: the quarter is not a nickel. The other coin is.
A farmer must take a fox, a chicken, and grain across a river. The boat carries the farmer and one item. Without the farmer, the fox cannot stay with the chicken, or the chicken with the grain. How can everything cross?
Good to know: Only the farmer can row the boat, and it holds the farmer plus at most one item. Trouble happens only on a bank the farmer has left, never during a crossing.
Hint 1
Which one is the troublemaker that cannot be left with either of the others?
Hint 2 (more help)
The boat can carry something back to the start, too.
Reveal the answer
Take the chicken across. Go back alone. Take the fox across. Bring the chicken back. Take the grain across. Go back alone. Take the chicken across.
The chicken is the problem: it cannot be left with the fox or with the grain, so it crosses first. Then the farmer takes the fox over, brings the chicken back, swaps it for the grain, and fetches the chicken last. Taking the grain before the fox works too. Seven trips is the fewest possible.
Anna, Beth, and Cara each have a different pet: cat, dog, or fish. Anna has neither cat nor dog. Beth does not have the dog. Who has each pet?
Good to know: Each girl has exactly one pet, and no two girls have the same kind of pet.
Hint 1
Start with the person the riddle tells you the most about.
Hint 2 (more help)
Anna has neither the cat nor the dog, so only one pet is left for her.
Reveal the answer
Anna has the fish, Beth has the cat, and Cara has the dog.
Anna has neither the cat nor the dog, so she has the fish. Beth does not have the dog, and the fish is taken, so Beth has the cat. That leaves the dog for Cara.
Eight balls look identical, but one is heavier. Using a balance scale, how can you always find it in no more than two weighings?
Good to know: Exactly one ball is heavier, and the other seven weigh the same. The balance scale only shows which side is heavier, or that the two sides are equal.
Hint 1
Think about splitting them into more than two groups.
Hint 2 (more help)
Put 3 on each side for the first weighing. What can each result tell you?
Reveal the answer
Weigh three against three. If they balance, weigh the last two against each other. If one side is heavier, take two balls from that side and weigh one against the other; if they balance, the third ball from that side is the heavy one.
Weighing 3 against 3 splits the eight balls into three groups: one pan, the other pan, and the 2 set aside. If the scale balances, the heavy ball is one of the 2 set aside, and one more weighing finds it. If one side is heavier, the heavy ball is one of those 3: weigh 1 against 1, and if they balance, it is the third. Either way, two weighings are enough.
A drawer has three red socks and three blue socks. Without looking, how many socks must you take out to be sure of getting two of the same color?
Good to know: You take socks out one at a time without looking, and any two socks of the same color count as a pair.
Hint 1
Think about the unluckiest way the socks could come out.
Hint 2 (more help)
Your first two socks might be one red and one blue. What about the next sock?
Reveal the answer
Three.
The first two socks might not match: one red and one blue. The third sock has to be red or blue, so it always matches one of them. Three socks are always enough, and two are not.