Can someone please explain to me how Fred died?

He was fighting alongside Percy and then Bellatrix Lestrange came and shot a killing curse at him. He was laughing because Percy made a joke saying that he had resigned from his job at the Ministry. It is said that 'He lay there, the ghost of his last laugh still etched on his face.'

While hes Fighting with Percy ,The wall breaks apart and sadly Fred dies.Later in the book it Belatrix basically admits to killing him like while she and Molly start dueling she said,
"What will happen to the children when I've killed you.When Mummy's gone the sames way as Freddie?"It states that on PG.736;Flaw In the Plan

i think falling stones!

Fred was killed in an explosion caused by death eater Augustus Rookwood according to J.K. Rowling.