Their reactivity increases going down the group: while lithium, sodium and potassium merely burn in air, rubidium and caesium are pyrophoric (spontaneously catch fire in air).
ðɛɹ ɹɪɐktˈɪvᵻɾi ˈɪŋkɹiːsᵻz ɡˌoʊɪŋ dˌaʊn ðə ɡɹˈuːp: wˌaɪl lˈɪθiəm, sˈoʊdiəm ænd pətˈæsiəm mˈɪɹli bˈɜːn ɪn ˈɛɹ, ɹuːbˈɪdiəm ænd sˈiːsiəm ɑːɹ pˌaɪɹoʊfˈoːɹɪk spɑːntˈeɪniəsli kˈætʃ fˈaɪɚɹ ɪn ˈɛɹ.