Missing: Stamp

I took Liam and Jonas to the library today for story time. While we were there they each got a stamp on their hand. Jonas was especially proud of his and made sure to show Nate when he got home from school. Tonight after their bath Jonas realized his stamp was gone. He got really upset and could hardly tell me what was wrong (Liam had to translate for me). I tried to comfort him and explain that it washed off in his bath. So a little while later when we were in the bathroom brushing teeth Jonas started looking in the bathtub and asked where the stamp was!