Hook and content bijections

We provide a bijective proof of the identity ∑x∈λ(h(x)2−c(x)2)=|λ|2 where λλ is an integer partition, h(x)h(x) is the hook number of the cell x∈λx∈λ, and c(x)c(x) is the content of xx. A closely related identity is also proved bijectively.