Kids Riddles

Question: My boots can fit though the green glass doors but my shoes cannot. Sea weed can fit but algae cannot. A tree can fit but a forest cannot. The moon can fit but a feather cannot. Why is this?

Question: If you have a cake, how many pieces of cake can you form with 3 straight cuts?

Question: Draw, fire, or fill me;
I'm still empty.

What am I?

Question: what did the postman say to the mailbox?

Question: What do you call 2 Us?